TABLE OF CONTENTS

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QUESTIONS AND ANSWERS ABOUT THE MERGER

Q:	What will happen to Kerman StateRedwood Empire Bancorp and National Bank of the Redwoods in this merger?

A:

ImmediatelyRedwood Empire Bancorp will merge with Westamerica Bancorporation, with Westamerica Bancorporation as the surviving corporation. Promptly after that National Bank of the merger, Kerman State BankRedwoods will merge with Westamerica Bank. The resulting bank will continue under the name “Westamerica Bank” as a wholly owned subsidiary of Westamerica.Westamerica Bancorporation. The existing branches of Kerman StateNational Bank of the Redwoods willeitherbecome or be consolidated with branches of Westamerica Bank.

A:	Simply indicate on your proxy card how you want to vote and then sign and mail your proxy card in the enclosed return envelope as soon as possible so that your shares may be represented at the Kerman State BankRedwood Empire Bancorp special meeting.

A:	Your broker will not vote your shares for you unless you provide instructions to your broker on how to vote. It is important therefore that you follow the directions provided by your broker regarding how to instruct your broker to vote your shares. If you fail to instruct your broker how to vote your shares, the effect will be the same as a vote against the merger agreement.

A:

Yes. You may change your vote at any time before your proxy is voted at the Special Meeting.special meeting. If your shares are held in your name you may do this in one of three ways. First, you may send a written notice stating that you would like to revoke your proxy. Second, you may complete and submit a new proxy card. If you choose either of these two methods, youRedwood Empire Bancorp must submitreceive your notice of revocation or your new proxy card to Kerman State BankRedwood Empire Bancorp at the address at the top of the Notice of Special Meeting in time so as to receive it prior to the vote at the special meeting. Third, you may attendrevoke your proxy before it is exercised by attending the special meeting and vote in person if you tell the Secretary that you wantelecting to cancel your proxy and vote in person. Simply attending the Kerman State Bank special meeting, however, will not revoke your proxy. If you have instructed a broker to vote your shares, you must follow directions received from your broker to change your vote or to vote at the Kerman State BankRedwood Empire Bancorp special meeting.

A:	No. After the merger is completed, we will send you written instructions for exchanging your stock certificates.

A:	We are working toward completing this merger as quickly as possible. We currently expect, subject to receiving required regulatory approvals, to complete this merger in mid-2002.the fourth quarter of 2004 or the first quarter of 2005.

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SUMMARY

     This summary, together with the “Questions and Answers” on the preceding pages, highlights important selected information from this proxy statement/prospectus. To understand the merger fully and for a more complete description of the legal terms of the merger, you should read carefully this entire document and the other information available to you. We have included page references in parentheses to direct you to a more complete description of the topics presented in this summary.

Kerman State BankRedwood Empire Bancorp shareholders will receive approximately 0.2731$28.74, consisting of a share$11.49 in cash and $17.25 in shares of Westamerica Bancorporation common stock in the merger, but this ratioamount is subject to possible adjustment (page 25)44)

     WhenIf the merger is completed, you will receive approximately 0.2731 of a share of Westamerica common stock for each share of Kerman State BankRedwood Empire Bancorp common stock that you hold, subjectwill be converted into the right to certain possible adjustments.receive merger consideration of $28.74, consisting of $11.49 in cash and $17.25 in shares of Westamerica Bancorporation common stock, valued at its average closing price for the 20 trading days ending three business days before the effective date of the merger. The merger consideration may be reduced by up to $0.30 per share (allocated proportionately between cash and stock) if bank regulators require Westamerica Bancorporation or Redwood Empire Bancorp to divest deposits in Lake County as a condition of approving the merger. For example, based on an assumed Westamerica average closing price of $55.25 (the average closing price for the 20 trading days ended October 11, 2004) and a resulting exchange ratio of 0.3122, if you holdheld 100 shares of Kerman State BankRedwood Empire Bancorp common stock, you willwould have the right to receive 27.31$1,149 in cash and 31.22 shares of Westamerica Bancorporation common stock in the merger (assuming no adjustments). You will receive cash instead of fractional shares. Therefore, you would only receive 2731 shares of Westamerica Bancorporation common stock. You would also receive a check in an amount equal to 0.310.22 of a share multiplied by an average closing price of the Westamerica Bancorporation common stock. Thestock on account of your fractional share, or $12.16 based on an average closing price of $55.25.

The following table shows the effect on the exchange ratio and the value of the merger consideration of changes in the average closing price of Westamerica Bancorporation common stock (rounded to the nearest cent):

     The merger agreement may be terminated by the board of directors of Redwood Empire Bancorp if Westamerica’s average closing price is less than $40.44. The merger agreement may be terminated by the board of directors of Westamerica Bancorporation if Westamerica Bancorporation’s average closing price is greater than $60.66.

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The merger consideration will be determined basedreduced if Westamerica Bancorporation is required to divest deposits in Lake County as a condition of any governmental approval of the merger. Any reduction will be allocated proportionately between the stock portion and the cash portion. The following table shows the effect of deposit divestitures on the value of the merger consideration (assuming no adjustment on account of a change in Westamerica’s average closing pricesprice).

The following table shows the effect on the exchange ratio and the value of the merger consideration of changes in the average closing price of Westamerica Bancorporation common stock, assuming a required divestiture of more than $30 million in deposits in Lake County:

If Westamerica Bancorporation is required to divest at least $10 million but not more than $30 million in deposits, the reductions in the exchange ratio, value of the stock portion and the total merger consideration would be proportionately less. For additional detail on eachthe effect of the 20 consecutive trading days prior to two business days before the closing date, the datethese possible adjustments, see “The merger agreement — Conversion of Redwood Empire Bancorp common stock” on which the transactions relating to the merger are completed.

     The exchange ratio (the number of shares of Westamerica common stock into which each share of Kerman State Bank common stock will be converted at closing) is a function of several factors, including Kerman State Bank’s adjusted equity (as defined in the merger agreement), amount of performing loans and Significant Liabilities (as defined in the merger agreement). The exchange ratio may be reduced if:

	•	Kerman State Bank’s performing loans before closing are less than $60 million;
	•	Kerman State Bank’s allowance for loan losses before closing, determined under Westamerica’s methodology, is greater than $3.2 million;
	•	Kerman State Bank incurs certain obligations defined as Significant Liabilities, which include new unapproved leases and other contractual obligations; capital expenditures above $25,000; contingent liabilities related to hazardous materials; new or accelerated severance or pension obligations; merger-related expenses above $470,000; and certain loan loss provisions and write-downs of foreclosed property; or
	•	The average closing price for Westamerica common stock over the 20 trading days ending two business days before the closing is greater than $41.76.

     The exchange ratio may be increased if:

	•	Kerman State Bank’s allowance for loan losses before closing, determined under Westamerica’s methodology, is less than $2.4 million; or
	•	Westamerica’s average closing price is less than $33.80.

The adjustments that will be made on account of the forgoing factors are interrelated and are described in Section 2.1(a) of the merger agreement.page 44.

Comparative market price dataMarket Price Data

     Westamerica Bancorporation’s common stock is quotedtraded on Nasdaq. Kerman State Bankthe Nasdaq National Market under the symbol “WABC.” Redwood Empire Bancorp common stock is thinly traded inon the over-the-counter market, is quoted inNasdaq National Market under the “Electronic Bulletin Board” and is not quoted on Nasdaq.symbol “REBC.”

     The following table presents historical per share market values for Westamerica Bancorporation common stock and Kerman State BankRedwood Empire Bancorp common stock and the equivalent pro forma market values (i) on February 22, 2002,August 25, 2004, the last trading day prior to public announcement of the merger and (ii) on [                    , 2002]2004]. The historical

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(1)	The historical market price for Kerman State Bank common stock is based on information provided to management by various sources, but not verified by Kerman State Bank. Kerman State Bank has not been a party to any trades reflected in that information.

Westamerica cannot assure you that the price of its stock will be the same or greater than the prices shown in the table at the time of the merger, or at any time after the completion of the merger. merger, so the market value of Westamerica

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Once Kerman State BankRedwood Empire Bancorp is acquired by Westamerica Bancorporation, there will be no further public market for Kerman State Bank’sRedwood Empire Bancorp’s common stock. Westamerica Bancorporation common stock will continue to be traded on Nasdaq.the Nasdaq National Market.

The merger is intended to be a tax-free transaction in which Kerman State BankRedwood Empire Bancorp shareholders will not recognize gain or loss on the stock portion of the merger consideration (page 21)38)

     The merger is intended to be a tax-free reorganization so that no gain or loss will be recognized by either Westamerica Bancorporation or Kerman State BankRedwood Empire Bancorp or their respective shareholders for federal income tax purposes except foron the stock portion of the merger consideration. Redwood Empire Bancorp shareholders may recognize gain but not loss on the receipt of the cash portion of the merger consideration.

Redwood Empire Bancorp’s board recommends shareholder approval (pages 18, 22)

Redwood Empire Bancorp’s board of directors believes that Kerman State Bankthe merger is in the best interests of Redwood Empire and its shareholders will receive insteadand has unanimously approved the merger agreement. Redwood Empire Bancorp’s board unanimously recommends that you vote “FOR” approval of fractionalthe merger agreement.

Vote Required (page 19)

     Approval of the merger by Redwood Empire Bancorp shareholders requires the affirmative vote of the holders of a majority of the outstanding shares. The directors and executive officers of Redwood Empire Bancorp and their affiliates together held approximately 1.0% of Redwood Empire’s outstanding common stock as of the record date. You should be aware that Redwood Empire Bancorp shareholders holding approximately 49.6% of the outstanding shares or cash received by Kerman State Bankhave agreed to vote in favor of the merger and have given Westamerica Bancorporation irrevocable proxies to vote their shares in favor of the merger and that directors of Redwood Empire, who in the aggregate own approximately 1.0% of the outstanding shares, have also agreed to vote their shares in favor of the merger. Therefore shareholder approval of the merger is effectively assured.

The merger does not require the approval of the shareholders who perfect their dissenter’s rights.of Westamerica Bancorporation.

Financial advisor giveshas given an opinion that consideration is fair to Kerman State BankRedwood Empire Bancorp shareholders (page 14)25)

     In deciding to approve the merger, the Kerman State BankRedwood Empire Bancorp board of directors considered the opinion of its financial advisor, James H. Avery Company,Hovde Financial LLC, dated as of February 5, 2002,August 13, 2004, about the fairness of the merger to Kerman State BankRedwood Empire Bancorp shareholders from a financial point of view. This opinion is attached as AppendixAnnex B to this proxy statement/prospectus. We encourage you to read it carefully. Under an agreement with Kerman State Bank, James H. Avery CompanyRedwood Empire Bancorp, Hovde Financial LLC will receive a fee of approximately $237,000,$1,530,000, subject to adjustment depending on the actual value of the merger consideration as of closing, when the merger closes.

Kerman State Bank’s board recommends shareholder approval (page 13)

Kerman State Bank’s board of directors believes that the merger is in the best interests of Kerman State Bank and its shareholders and has unanimously approved the merger agreement. Kerman State Bank’s board recommends that you vote “FOR” approval of the merger agreement.

Shareholders of Westamerica and Kerman State Bank have different rights (page 65)

The rights of holders of Westamerica common stock are different in some respects from the rights of holders of Kerman State Bank common stock. You should consider these differences when voting on the merger.

Dissenters’ rights of appraisal (page 2240 and AppendixAnnex C)

     InsteadYou will be given the opportunity to exercise dissenters’ rights in accordance with certain procedures specified under California law, provided that demands are made for payment with respect to 5% or more of receiving Westamericathe outstanding shares of Redwood Empire Bancorp common stock. This means that shareholders who vote against the merger may make a written demand to Redwood Empire Bancorp for payment in cash of the “fair market value” of their shares. Redwood Empire Bancorp must receive the demand no later than the date of the special meeting. To exercise dissenters’ rights, you must vote shares against the merger; it is not sufficient

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Benefits to certain officers and directors in the merger (page 20)35)

     In considering the recommendation of the board of directors of Kerman State BankRedwood Empire Bancorp to approve the merger agreement, you should be aware that certain executive officers and directors of Kerman State BankRedwood Empire Bancorp have certain interests in, and will receive benefits as a consequence of, the merger that are different from the benefits to Kerman State BankRedwood Empire Bancorp shareholders generally. These interests include:

	•	Certainvesting of all outstanding stock options, which, based on anticipated holdings as of December 1, 2004 and merger consideration of $28.74, would result in an estimated aggregate benefit to executive officers and directors of Kerman State Bank will receive fully vested options to acquire Westamerica common stock in place of existing vestedapproximately $508,150; and unvested options to acquire Kerman State Bank common stock;
	•	Certainpotential severance benefits available to executive officers of Kerman State Bank will be eligible to receiveand directors under various severance policies, change in control agreements and salary continuation agreements, which could result in cash severance payments of up to twelve months salaryexecutive officers and in certain instances, bonuses if Westamerica does not make an offer to retain them as employees in comparable positions or if such officers aredirectors, assuming all were terminated during the twelve month periodimmediately following the merger.merger, of approximately $1,670,055 in the aggregate.

Shareholders of Westamerica Bancorporation and Redwood Empire Bancorp have different rights (page 69)

The rights of holders of Westamerica Bancorporation common stock are different in some respects from the rights of holders of Redwood Empire Bancorp common stock. You should consider these differences when voting on the merger.

Information regarding Westamerica Bancorporation and Kerman State BankRedwood Empire Bancorp (pages 3321, 54 and 42)64)

Westamerica Bancorporation

     Westamerica Bancorporation is a bank holding company registered under the Bank Holding Company Act of 1956. Westamerica Bancorporation was incorporated under the laws of the State of California as “Independent Bankshares Corporation” on February 11, 1972. It is the parent of Westamerica Bank and Community Banker Services Corporation. Westamerica Bancorporation has 9088 branches throughout northern and central California. Westamerica isBancorporation as of June 30, 2003, was the 13th14th largest bank and 19th18th largest depository institution, measured by deposits, in California. At December 31, 2001,June 30, 2004 it had total assets of $3.9$4.6 billion, deposits of approximately $3.2$3.5 billion and shareholders’ equity of $314$329.8 million.

Kerman State Bank

     Kerman State BankRedwood Empire Bancorp is a state banking corporation licensed as a commercial bank byholding company registered under the California DepartmentBank Holding Company Act of Financial Institutions (“DFI”).1956. It was originally incorporated in 1983. It is headquartered1988 for the purpose of becoming the holding company of National Bank of the Redwoods, a national banking association founded in Kerman, Fresno County, California. Kerman State Bank1985. Redwood Empire Bancorp conducts a general commercial banking business through its main office in Santa Rosa, California, five retail branches located in Sonoma County, California, one retail branch located in Mendocino County, California, and two branchesone retail branch located in Firebaugh and Easton,Lake County, California. At December 31, 2001, Kerman State BankJune 30, 2004, Redwood Empire Bancorp had assets of

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Special shareholders meeting to be held on [meeting date], 20022004 (page 10)18)

     We will hold the special meeting of shareholders of Redwood Empire Bancorp at [meeting time] on Thursday, [meeting date], 2002,2004, at Kerman State Bank, 306 South MaderaRedwood Empire Bancorp, 111 Santa Rosa Avenue, Kerman,Santa Rosa, California 93630.95404. At the special meeting, you will be asked (1) to approve the merger agreement and (2) to consider such other business as may properly come before the meeting.

Record date set at April 15, 2002;[record date], 2004; vote required for approval of merger (page 11)19)

     You can vote at the special meeting if you owned Kerman State BankRedwood Empire Bancorp common stock at the close of business on April 15, 2002.[record date], 2004. Holders of a majority of the outstanding shares of Kerman State BankRedwood Empire Bancorp common stock must vote to approve the merger agreement in order for the merger to occur.

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Conditions that must be satisfied for the merger to occur (page 30)51)

     We will not complete the merger unless a number of conditions are met. These include:

	•	approval of the merger agreement by Kerman State BankRedwood Empire Bancorp shareholders,
	•	receipt of all required regulatory approvals,approvals;
	•	absence of material adverse changes in the parties, unless waived; and
	•	absencesatisfaction by Redwood Empire Bancorp of any orders suspending the effectivenesscertain financial conditions relating to its levels of the registration statement filedperforming loans, noninterest-bearing deposits and shareholders’ equity, unless waived by Westamerica to register the shares to be issued to Kerman State Bank shareholders.Bancorporation.

Regulatory approvals we must obtain for the merger to occur (page 19)34)

     The merger requires the prior approval of the Board of Governors of the Federal Reserve System (the “FRB”) under the Bank Merger Act and of the DFI.California Department of Financial Institutions under the California Financial Code. The FRB may require Westamerica Bancorporation or Redwood Empire Bancorp to divest deposits in Lake County as a condition to its approval. If such divestiture is required, the merger consideration could be reduced by up to $0.30 per share.

Westamerica Bancorporation to use purchase accounting treatment (page 21)38)

     Westamerica Bancorporation will account for the merger as a purchase. Under the purchase accounting method, Westamerica Bancorporation will revalue on its books the assets and liabilities of Kerman State BankRedwood Empire Bancorp at their fair market values, and the amount by which the aggregate merger consideration exceeds the net fair values of the assets of Kerman State BankRedwood Empire Bancorp will be recorded as intangible assets calledintangibles such as goodwill or core deposit intangible.

Termination of the merger agreement (page 32)52)

     The merger agreement may be terminated before completion as follows:

	•	by the mutual consent of Kerman State BankRedwood Empire Bancorp and Westamerica;Westamerica Bancorporation;
	•	by either party if the conditions to its obligations have not been fulfilled;met by May 25, 2005;
	•	by either party if the shareholders of Redwood Empire Bancorp fail to approve the merger agreement at the Special Meeting;
	•	by either party if the other fails to comply materially with its obligations under the merger agreement;agreement and fails to cure within 60 days;
	•	by either party if any necessary governmental approval has been denied;
	•	by Westamerica Bancorporation if someone other than Westamerica enters intoRedwood Empire Bancorp materially breaches its obligation not to solicit or encourage a different transaction for the acquisition of Kerman State Bank;Redwood Empire Bancorp, and, in the

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		event of such termination, Redwood Empire Bancorp will be liable to Westamerica Bancorporation for a break-up fee of $4,500,000;
	•	by Westamerica Bancorporation if itsthe board of directors determines that it is not advisableof Redwood Empire Bancorp fails to consummaterecommend adoption of the merger by reasonagreement at the special meeting, and, in the event of any material legal impediment; orsuch termination, Redwood Empire Bancorp will be liable to Westamerica Bancorporation for a break-up fee of $4,500,000;
	•	by Kerman State BankRedwood Empire Bancorp if Westamerica entersits board of directors authorizes it to enter into a superior transaction with someone providing for the acquisition of Redwood Empire Bancorp, and, in the event of such termination, Redwood Empire Bancorp will be liable to Westamerica which does not provideBancorporation for the assumptiona break-up fee of $4,500,000;
	•	by either party if Westamerica Bancorporation solicits or encourages an acquisition of Westamerica Bancorporation that would materially interfere with its obligations under the merger agreement.agreement, and, in the event of such termination, Westamerica Bancorporation will be liable to Redwood Empire Bancorp for a break-up fee of $4,500,000;
	•	by Westamerica Bancorporation if the Westamerica average closing price is greater than $60.66; and
	•	by Redwood Empire Bancorp if the Westamerica average closing price is less than $40.44.

     If Westamerica terminates the merger agreement as a result of Kerman State Bank’s failure to obtain a fairness opinion or shareholder approval, or due to Kerman State Bank’s breach of the merger agreement, then Kerman State Bank must pay Westamerica a fee of $250,000.

     If Kerman State Bank terminates the merger agreement as a result of its failure to obtain a fairness opinion, then Kerman State Bank must pay Westamerica a fee of $250,000. If Kerman State Bank terminates this Agreement due to a breach of the merger agreement by Westamerica Bancorporation or Westamerica, then Westamerica shall pay Kerman State Bank a fee of $250,000.

     If Kerman State Bank enters into or solicits a competing transaction, Westamerica may terminate the merger agreement and Kerman State Bank will be liable for a break-up fee of $1,500,000. If Westamerica enters into a competing transaction that precludes Westamerica from completing the merger, Kerman State Bank may terminate the merger agreement and Westamerica will be liable for a break-up fee of $1,000,000.

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Selected Financial Information aboutAbout Westamerica Bancorporation

     The following table presents selected supplemental historical financial information for Westamerica Bancorporation for each of the five years in the period ended December 31, 2001.2003, and the six-month periods ended June 30, 2004 and 2003. This information has been derived from and should be read in conjunction with the supplemental consolidated financial statements of Westamerica Bancorporation, including the notes to such financial statements, which are incorporated by reference elsewhere in this document.