SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

Form 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d) of the

Securities Exchange Act of 1934

Date of Report (Date of earliest event

Reported): September 30, 2003

Credit Suisse First Boston Mortgage Securities Corp.,

CSFB Series 2003-AR20,

Mortgage-Backed Pass-Through Certificates, Series 2003-AR20


CREDIT SUISSE FIRST BOSTON MORTGAGE SECURITIES CORP.
(Exact name of registrant as specified in its charter)

Delaware	333-100669	13-3320910
(State or Other Jurisdiction of Incorporation)	(Commission File Number)	(I.R.S. Employer Identification No.)


11 Madison Avenue
New York, New York 10010
(Address of Principal Executive Offices) (Zip Code)

Registrant’s telephone number, including area code (212) 325-2000

Item 5.

Other Events.

Credit Suisse First Boston Mortgage Securities Corp. (the “Company”) entered into a Pooling and Servicing Agreement dated as of July 1, 2003 (the “Agreement”) among the Company, in its capacity as depositor (in such capacity, the “Depositor”), DLJ Mortgage Capital, Inc. as a seller (in such capacity, a “Seller”), Chase Manhattan Mortgage Corporation in its capacity as master servicer (in such capacity, the “Master Servicer”), Washington Mutual Mortgage Securities Corp., in its capacity as a seller (in such capacity a “Seller”) and in its capacity as a servicer (in such capacity, a “Servicer”), Fairbanks Capital Corp., in its capacity as a servicer (in such capacity, a “Servicer”) and as special servicer (in such capacity, the “Special Servicer”), Bank One, National Association, as trustee (the “Trustee”) and JPMorgan Chase Bank, as trust administrator (the “Trust Administrator”). The Certificates were issued on July 30, 2003. The Pooling and Servicing Agreement is annexed hereto as Exhibit 99.1.

Item 7. Financial Statements, Pro Forma Financial Information and Exhibits.

(a)

Not applicable.

(b)

Not applicable.

(c)

Exhibits:

99.1

The Pooling and Servicing Agreement dated as of July 1, 2003, by and among the Company, the Sellers, the Servicers, the Special Servicer, the Master Servicer, the Trustee and the Trust Administrator.

SIGNATURES

Filings Made by the Registrant. The registrant has duly caused this form to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of New York, State of New York, on September 30, 2003.

CREDIT SUISSE FIRST BOSTON MORTGAGE

SECURITIES CORP.

By: /s/ Craig Eckes

Name: Craig Eckes

Title: Vice President

Exhibit Index

Exhibit

Page

99.1

Pooling and Servicing Agreement dated as of July 1, 2003,

by and among the Company, the Seller, the Servicers, the Master Servicer, the

Special Servicer, the Trustee and the Trust Administrator.

EXHIBIT 99.1

CSFB MORTGAGE BACKED PASS THR CERTS SERIES 2003 AR20 8-K