Indicate by check mark whether the registrant (1) has filed all reports required to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months (or for such shorter period that the registrant was required to file such reports), and (2) has been subject to such filing requirements for the past 90 days. Yesþ Noo

Indicate by check mark whether the registrant has submitted electronically and posted on its corporate Web site, if any, every Interactive Data File required to be submitted and posted pursuant to Rule 405 of Regulation S-T (§232.405 of this chapter) during the preceding 12 months (or for such shorter period that the registrant was required to submit and post such files). Yeso Noo

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, or a smaller reporting company. See the definitions of “large accelerated filer,” “accelerated filer” and “smaller reporting company” in Rule 12b-2 of the Exchange Act.

Indicate by check mark whether the registrant is a shell company (as defined in Rule 12b-2 of the Exchange Act). Yeso Noþ

     ERP Operating Limited Partnership (“ERPOP”), an Illinois limited partnership, was formed in May 1993 to conduct the multifamily residential property business of Equity Residential (“EQR”). EQR, a Maryland real estate investment trust (“REIT”) formed in March 1993, is an S&P 500 company focused on the acquisition, development and management of high quality apartment properties in top United States growth markets. EQR has elected to be taxed as a REIT.

     EQR is the general partner of, and as of September 30, 2010March 31, 2011 owned an approximate 95.3%95.5% ownership interest in ERPOP. EQR is structured as an umbrella partnership REIT (“UPREIT”) under which allAll of EQR’s property ownership, development and related business operations are conducted through ERPOP and its subsidiaries. References to the “Operating Partnership” include ERPOP and those entities owned or controlled by it. References to the “Company” mean EQR and the Operating Partnership.

     As of September 30, 2010,March 31, 2011, the Operating Partnership, directly or indirectly through investments in title holding entities, owned all or a portion of 471442 properties located in 1817 states and the District of Columbia consisting of 133,029127,711 apartment units. The ownership breakdown includes (table does not include various uncompleted development properties):

entities after consideration of any net operating losses.

     Deferred tax assets and liabilities are recognized for future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases. These assets and liabilities are measured using enacted tax rates for which the temporary differences are expected to be recovered or settled. The effects of changes in tax rates on deferred tax assets and liabilities are recognized in earnings in the period enacted. The Operating Partnership’s deferred tax assets are generally the result of tax affected amortization of goodwill, differing depreciable lives on capitalized assets and the timing of expense recognition for certain accrued liabilities. As of September 30, 2010,March 31, 2011, the Operating Partnership has recorded a deferred tax asset of approximately $42.5$38.7 million, which is fully offset by a valuation allowance due to the uncertainty in forecasting future TRS taxable income.

     Other

     Effective January 1, 2010, in an effort to improve financial standards for transfers of financial assets, more stringent conditions for reporting a transfer of a portion of a financial asset as a sale (e.g. loan participations) are required, the concept of a “qualifying special-purpose entity” and special guidance for guaranteed mortgage securitizations are eliminated, other sale-accounting criteria is clarified and the initial measurement of a transferor’s interest in transferred financial assets is changed. This does not have a material effect on the Operating Partnership’s consolidated results of operations or financial position.

     Effective January 1, 2010, the analysis for identifying the primary beneficiary of a Variable Interest Entity (“VIE”) has been simplified by replacing the previous quantitative-based analysis with a framework that is based more on qualitative judgments. The analysis requires the primary beneficiary of a VIE to be identified as the party that both (a) has the power to direct the activities of a VIE that most significantly impact its economic performance and (b) has an obligation to absorb losses or a right to receive benefits that could potentially be significant to the VIE. For the Operating Partnership, this includes only its consolidated development partnerships as the Operating Partnership provides substantially all of the capital for these ventures (other than third party mortgage debt, if any). For the Operating Partnership, these requirements affected only disclosures and had no impact on the Operating Partnership’s consolidated results of operations or financial position. See Note 6 for further discussion.

     The Operating Partnership is required to make certain disclosures regarding noncontrolling interests in consolidated limited-life subsidiaries. The Operating Partnership is the controlling partner in various consolidated partnerships owning 2423 properties and 5,0224,828 apartment units and various completed and uncompleted development properties having a noncontrolling interest book value of $8.5$7.7 million at September 30, 2010.March 31, 2011. Some of these partnership agreements contain provisions that require the partnerships to be liquidated through the sale of their assets upon reaching a date specified in each respective partnership agreement. The Operating Partnership, as controlling partner, has an obligation to cause the property owning partnerships to distribute the proceeds of liquidation to the Noncontrolling Interests in these Partially Owned Properties only to the extent that the net proceeds received by the partnerships from the sale of their assets warrant a distribution based on the partnership agreements. As of September 30, 2010,March 31, 2011, the Operating Partnership estimates the value of Noncontrolling Interest distributions would have been approximately $62.2$64.1 million (“Settlement Value”) had the partnerships been liquidated. This Settlement Value is based on estimated third party consideration realized by the partnerships upon disposition of the Partially Owned Properties and is net of all other assets and liabilities, including yield maintenance on the mortgages encumbering the properties, that would have been due on September 30, 2010March 31, 2011 had those mortgages been prepaid. Due to, among other things, the inherent uncertainty in the sale of real estate assets, the amount of any potential distribution to the Noncontrolling Interests in the Operating Partnership’s Partially Owned Properties is subject to change. To the extent that the partnerships’ underlying assets are worth less than the underlying liabilities, the Operating Partnership has no obligation to remit any consideration to the Noncontrolling Interests in these Partially Owned Properties.

     Effective January 1, 2010, companies are required to separately disclose the amounts of significant transfers of assets and liabilities into and out of Level 1, Level 2 and Level 3 of the fair value hierarchy and the reasons for those transfers. Companies must also develop and disclose their policy for determining when transfers between levels are

material effect on itsthe Operating Partnership’s consolidated results of operations or financial position. See Note 11 for further discussion.

     Effective January 1, 2009, issuers of certain convertible debt instruments that may be settled in cash on conversion were required to separately account for the liability and equity components of the instrument in a manner that reflects each issuer’s nonconvertible debt borrowing rate. As the Operating Partnership is required to apply this retrospectively, the accounting for the Operating Partnership’s $650.0 million ($482.5 million outstanding at September 30, 2010)March 31, 2011) 3.85% convertible unsecured notes that were issued in August 2006 and mature in August 2026 was affected. The Operating Partnership recognized $13.9$4.6 million and $15.5$4.6 million in interest expense related to the stated coupon rate of 3.85% for the nine monthsquarters ended September 30,March 31, 2011 and 2010, and 2009, respectively. The amount of the conversion option as of the date of issuance calculated by the Operating Partnership using a 5.80% effective interest rate was $44.3 million and is being amortized to interest expense over the expected life of the convertible notes (through the first put date on August 18, 2011). Total amortization of the cash discount and conversion option discount on the unsecured notes resulted in a reduction to earnings of approximately $5.8$2.0 million and $7.2$1.9 million, respectively, or $0.02$0.01 per Unit and $0.02$0.01 per Unit, respectively, for the nine monthsquarters ended September 30,March 31, 2011 and 2010, and 2009, and is anticipated to result in a reduction to earnings of approximately $7.8$5.0 million or $0.03$0.02 per Unit during the full year of 2010 assuming the Operating Partnership does not repurchase any additional amounts of this debt.2011. In addition, the Operating Partnership decreased the January 1, 2009 balance of retained earnings (included in general partner’s capital) by $27.0 million, decreased the January 1, 2009 balance of notes by $17.3 million and increased the January 1, 2009 balance of paid in capital (included in general partner’s capital) by $44.3 million. The carrying amount of the conversion option remaining in paid in capital (included in general partner’s capital) was $44.3 million at both September 30, 2010March 31, 2011 and December 31, 2009.2010. The unamortized cash and conversion option discounts totaled $6.9$3.0 million and $12.8$5.0 million at September 30, 2010March 31, 2011 and December 31, 2009,2010, respectively.

     The following tables present the changes in the Operating Partnership’s issued and outstanding “Units” (which includes OP Units and Long-Term Incentive Plan (“LTIP”) Units) and in the limited partners’ Units for the nine monthsquarter ended September 30, 2010:March 31, 2011:

     EQR has a share repurchase program authorized by the Board of Trustees under which it has authorization to issue an additional 12.4repurchase up to $464.6 million of its shares under the ATM program as of September 30, 2010.

     During the nine monthsquarter ended September 30, 2010,March 31, 2011, the Operating Partnership acquired all of its partner’s interest in two partially owned properties consisting of 432 units, one partially owned development project and one partially owned land parcel for $0.7 million. The Operating Partnership also increased its ownership in anotherconsolidated partially owned property through the buyoutconsisting of certain equity interests. One partially owned property buyout was funded through the issuance of 1,129 OP Units valued at $50,000.404 apartment units for $0.5 million. In conjunction with these transactions,this transaction, the Operating Partnership reduced paid in capital (included in general partner’s capital) by $1.3 million, other liabilities by $0.2 million and Noncontrolling Interests – Partially Owned Properties by $0.5 million.

     The Limited Partners of the Operating Partnership as of September 30, 2010March 31, 2011 include various individuals and entities that contributed their properties to the Operating Partnership in exchange for OP Units, as well as the equity positions of the holders of LTIP Units. Subject to certain exceptions (including the “book-up” requirements of LTIP Units), Limited Partners

     The following table summarizes the carrying amounts for the Operating Partnership’s investment in real estate (at cost) as of September 30, 2010March 31, 2011 and December 31, 20092010 (amounts in thousands):