POS EX

As filed with the Securities and Exchange Commission on September 22, 2010

Registration No. 333-166810

UNITED STATES SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

POST-EFFECTIVE

Amendment No. 1

to

Form S-11

REGISTRATION STATEMENT
UNDER
THE SECURITIES ACT OF 1933
OF SECURITIES OF CERTAIN REAL ESTATE COMPANIES

CoreSite Realty Corporation

(Exact name of registrant as specified in governing instruments)

1050 17th Street, Suite 800

Denver, CO 80265

(866) 777-2673

(Address, including zip code, and telephone number, including area code, of registrant’s principal executive offices)

Thomas M. Ray
President & Chief Executive Officer
CoreSite Realty Corporation
1050 17th Street, Suite 800
Denver, CO 80265

(866) 777-2673

(Name, address, including zip code, and telephone number, including area code, of agent for service)

Copies to:

Raymond Y. Lin	Edward J. Schneidman
Patrick H. Shannon	John P. Berkery
Latham & Watkins LLP	Mayer Brown LLP
885 Third Avenue	1675 Broadway
New York, New York 10022	New York, New York 10019
(212) 906-1200	(212) 506-2500

Approximate date of commencement of proposed sale to the public:  As soon as practicable after this Registration Statement becomes effective.

If any of the Securities being registered on this Form are to be offered on a delayed or continuous basis pursuant to Rule 415 under the Securities Act, check the following box:  o

If this Form is filed to register additional securities for an offering pursuant to Rule 462(b) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.  o

If this Form is a post-effective amendment filed pursuant to Rule 462(c) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.  o

If this Form is a post-effective amendment filed pursuant to Rule 462(d) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.  þ

If delivery of the prospectus is expected to be made pursuant to Rule 434, check the following box.  o

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. (Check one):

Large accelerated filer o	Accelerated filer o	Non-accelerated filer þ	Smaller reporting company o
	(Do not check if a smaller reporting company)

The Registrant hereby amends this Registration Statement on such date or dates as may be necessary to delay its effective date until the Registrant shall file a further amendment which specifically states that this Registration Statement shall thereafter become effective in accordance with Section 8(a) of the Securities Act of 1933 or until this Registration Statement shall become effective on such date as the Commission, acting pursuant to said Section 8(a), may determine.

EXPLANATORY NOTE

This Post-Effective Amendment No. 1 to Form S-11 Registration Statement (333-166810) is filed pursuant to Rule 462(d) solely to add an exhibit to such Registration Statement. No changes have been made to Part I or Part II of the Registration Statement other than Item 36(b) of Part II as set forth below.

Item 36.   Financial Statements and Exhibits.

(b) Exhibits.  The following is a complete list of exhibits filed as part of the registration statement, which are incorporated herein:

Exhibit
Number			Description
	4	.1	Specimen certificate representing the Common Stock of CoreSite Realty Corporation.

II-1

SIGNATURES

Pursuant to the requirements of the Securities Act of 1933, as amended, we have duly caused this Post-Effective Amendment No. 1 to the Registration Statement on Form S-11 to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of Denver, State of Colorado, on the 22nd day of September, 2010.

CORESITE REALTY CORPORATION

By:  /s/  Thomas M. Ray

Name:     Thomas M. Ray

Title:  President

Pursuant to the requirements of the Securities Act of 1933, this Post-Effective Amendment No. 1 to the Registration Statement has been signed by the following persons in the capacities and on the dates indicated.

Signature	Title	Date
/s/  Thomas M. Ray Thomas M. Ray	President and Director (Principal Executive Officer)	September 22, 2010
/s/  Deedee M. Beckman Deedee M. Beckman	Treasurer and Chief Financial Officer (Principal Financial Officer and Principal Accounting Officer)	September 22, 2010

II-2

EX-4.1

Exhibit 4.1

COMMON STOCK

Core Site Realty Corporation SEE REVERSE FOR CERTAIN DEFINITIONSCUSIP 21870Q 10 5

COMMON STOCK INCORPORATED UNDER THE LAWSOF THE STATE OF MARYLAND

THISCERTIFIESTHATis the record holder ofFULLY PAID AND NON-ASSESSABLE SHARES OF COMMON STOCK, $0.01 PAR VALUE PER SHARE, OFSEE REVERSE FOR CERTAIN DEFINITIONSCUSIP 21870Q 10 5CoreSite Realty Corporationtransferable on the books of the Corporation by the holder hereof, in person or by duly authorized attorney, upon surrender of thisCertificate properly endorsed. This Certificate is not valid unless countersigned and registered by the Transfer Agent and Registrar.

Witness the facsimile signatures of the Corporation’s duly authorized officers. DATED:

CHIEF FINANCIAL OFFICER PRESIDENT AND CHIEF EXECUTIVE OFFICER

AUTHORIZED SIGNATURE

BY:

TRANSFER AGENT AND REGISTRAR AMERICAN STOCK TRANSFER & TRUST COMPANY, LLC(New York, NY)TRANSFER AGENT AND REGISTRAR

The following abbreviations, when used in the inscription on the face of this certificate, shall be construed as though they were written out in full according to applicable laws or regulations:

TEN COM—as tenants in commonTEN ENT—as tenants by the entiretiesJT TEN—as joint tenants with rightof survivorship and not astenants in commonAdditional abbreviations may also be used though not in the above list.For value received, hereby sell, assign and transfer untoTHE SIGNATURE TO THIS ASSIGNMENT MUST CORRESPOND WITHTHE NAME AS WRITTEN UPON THE FACE OF THE CERTIFICATE, INEVERY PARTICULAR, WITHOUT ALTERATION OR ENLARGEMENT, ORANY CHANGE WHATEVER.The following abbreviations, when used in the inscription on the face of this certificate, shall be construed as though theywere written out in full according to applicable laws or regulations:NotiCe:SharesPLEASE PRINT OR TYPEWRITE NAME AND ADDRESS INCLUDING ZIP CODE OF ASSIGNEEof the common stock represented by the within Certificate, and do hereby irrevocablyconstitute and appointto transfer the said stock on the books of the within-named Corporation with full power ofsubstitution in the premises.Dated,PLEASE INSERT SOCIAL SECURITY OR OTHERIDENTIFYING NUMBER OF ASSIGNEEPleaSe iNitial tHe aPProPriate SeleCtioN For tHiS ProoF:oK?aS iSoK WitH CHaNGeSMaKe CHaNGeS aND SeND aNotHer ProoFUNIF GIFT MIN ACT—......................... Custodian .........................(Cust) (Minor)under Uniform Gifts to MinorsAct         ..............................................................

Attorney

SiGNatUre(S) GUaraNteeD: THE SIGNATURE(S) MUST BE GUARANTEED BY AN ELIGIBLE GUARANTORINSTITUTION

(BANKS, STOCKBROKERS, SAVINGS AND LOAN ASSOCIATIONSAND CREDIT UNIONS WITH MEMBERSHIP IN AN APPROVED SIGNATURE

GUARANTEE MEDALLION PROGRAM), PURSUANT TO S.E.C. RULE 17Ad-15.