UNITED STATES
                       SECURITIES AND EXCHANGE COMMISSION

                             Washington, D.C. 20549


                                    FORM 10-K

[X]      Annual  report  pursuant  to  Section  13 or  15(d)  of the Securities
         Exchange   Act  of  1934   for   the    period  from  December 1, 2001
         (Commencement of Operations) to December 31, 2001

[  ]     Transition report pursuant to Section 13 or 15(d) of the
         Securities Exchange Act of 1934 for the transition period from   to


                        Commission File Number 333-61840-15

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

         Delaware                                            13-3320910
(State or other jurisdiction of                           I.R.S. Employer
incorporation or organization)                            Identification No.)

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

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


         CREDIT SUISSE FIRST BOSTON MORTGAGE SECURITIES CORP.
       CSFB MORTGAGE PASS-THROUGH CERTIFICATES, SERIES 2001-S31
       (Title of each class of securities covered by this Form)

Securities registered pursuant to Section 12(b) of the Act:  None
Securities registered pursuant to Section 12(g) of the Act:  None

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. Yes [X] No [ ]

Indicate by check mark if disclosure of delinquent  filers  pursuant to Item 405
of Regulation  S-K is not contained  herein,  and will not be contained,  to the
best of registrant's  knowledge,  in definitive proxy or information  statements
incorporated by reference in Part III of this Form 10-K or any amendment to this
Form 10-K. [X]

Documents incorporated by reference:  None



<PAGE>

CREDIT SUISSE FIRST BOSTON MORTGAGE SECURITIES CORP.
CSFB MORTGAGE PASS-THROUGH CERTIFICATES, SERIES 2001-S31
--------------------------------------------------------

PART I

Item 1.  Business

         Omitted.

Item 2.  Properties

         Omitted.

Item 3.  Legal Proceedings

     The Registrant is not aware of any material legal  proceeding  with respect
to, the Company, the Master Servicer, the Servicers,  the Trust Administrator or
the Trustee, in each case if applicable, as related to the Trust.

Item 4.  Submission of Matters to a Vote of Security Holders

No  matter  was  submitted  to a vote  or  consent  of  Holders  of the  Offered
Certificates during the fiscal year covered by this report.


PART II

Item 5.  Market for Registrant's Common Equity and Related Stockholder Matters

The Trust does not issue stock. There is currently no established secondary
market for the Certificates.  As of  December  31,  2002,  the number
of holders  of each  Class  of  Offered  Certificates was less than 300.


Item 6.  Selected Financial Data

         Omitted.

Item 7.  Management's Discussion and Analysis of Financial condition and
         Results of Operations

         Omitted.

Item 8.  Financial Statements and Supplementary Data

         Omitted.

Item 9.  Changes in and Disagreements with Accountants on Accounting and
         Financial Disclosure

There was no change of  accountants  or  disagreement  with  accountants  on any
matter of accounting principles or practices or financial disclosure.

                                      -2-

<PAGE>

PART III

Item 10. Directors and Executive Officers of the Registrant

         Omitted.

Item 11. Executive Compensation

         Omitted.

Item 12. Security Ownership of Certain Beneficial Owners and Management

         Omitted.


Item 13. Certain Relationships and Related Transactions

No reportable transactions have occurred.


PART IV

Item 14. Exhibits, Financial Statement Schedules and Reports on Form 8-K

(a) The following documents are filed as part of this report:

    (1) Financial Statements:
         Omitted.

    (2)  Financial Statement Schedules:
         Omitted.

    (3)  Exhibits:

   Annual  Servicers'  Statement of  Compliance,  filed as Exhibit 99.1 hereto.
   Annual Statement of  Independent  Accountants  Report for the Servicers,
   filed as Exhibit 99.2 hereto.

(b)   Reports on Form 8-K: The  following  Current  Reports on Form 8-K were
      filed by the Registrant during the last quarter of 2001.

      No reportable transactions have occurred.

(c)   Exhibits to this report are listed in Item (14)(a)(3) above.

                                      -3-
<PAGE>
Annual report  pursuant to Section 13 or 15(d) of the  Securities  Exchange
Act of 1934 for the fiscal year ended December 31, 2002


CREDIT SUISSE FIRST BOSTON MORTGAGE SECURITIES CORP.
CSFB MORTGAGE PASS-THROUGH CERTIFICATES, SERIES 2001-S31
--------------------------------------------------------

SIGNATURE

Pursuant to the  requirements of Section 13 or 15(d) of the Securities  Exchange
Act of 1934,  the  registrant  has duly  caused  this report to be signed on its
behalf by the undersigned thereunto duly authorized.

                                    CREDIT SUISSE FIRST BOSTON MORTGAGE
                                    SECURITIES CORP.



Date:  March 31, 2003               By: /s/ Matt Ruppel
                                    -----------------------------
                                    Name:  Matt Ruppel
                                    Title: Vice President



                                      -4-
<PAGE>


                                CERTIFICATION


         Re:      Credit Suisse First Boston Mortgage Securities Corp.,
                  CSFB Trust Series 2001-S31
                  CSFB Mortgage Pass-Through Certificates, Series 2001-S31

                  I, Matt Ruppel, certify that:

     1. I have reviewed this annual report on Form 10-K, and all reports on Form
8-K containing  distribution  and servicing  reports filed in respect of periods
included  in the year  covered  by this  annual  report,  of CSFB  Trust  Series
2001-S31 (the "Trust");

     2. Based on my knowledge,  the  information  in these  reports,  taken as a
whole, does not contain any untrue statement of a material fact or omit to state
a  material  fact  necessary  to make  the  statements  made,  in  light  of the
circumstances  under which such  statements  were made, not misleading as of the
last day of the period covered by this annual report;

     3. Based on my  knowledge,  the  distribution  information  required  to be
prepared  by the Trustee  based upon the  servicing  information  required to be
provided by each Servicer under the Pooling and Servicing  Agreement is included
in these reports;

     4. Based on my knowledge and upon the annual compliance statements included
in the report and required to be delivered to the Trustee in accordance with the
terms of the Pooling and Servicing  Agreement and based upon the review required
under the  Pooling  and  Servicing  Agreement,  and except as  disclosed  in the
report,  each  Servicer  has  fulfilled  its  obligations  under the Pooling and
Servicing Agreement; and

     5. The  reports  disclose  all  significant  deficiencies  relating to each
Servicer's  compliance with the minimum servicing standards based, in each case,
upon the report provided by an independent public accountant, after conducting a
review in compliance  with the Uniform Single  Attestation  Program for Mortgage
Bankers or similar standard as set forth in the Pooling and Servicing Agreement,
that is included in these reports.

     Capitalized terms used but not defined herein have the meanings ascribed to
them in the  Pooling  and  Servicing  Agreement,  dated  December  1,  2001 (the
"Pooling and Servicing  Agreement"),  among Credit Suisse First Boston  Mortgage
Securities Corp., as depositor (the "Depositor"), DLJ Mortgage Capital, Inc., as
seller (the  "Seller"),  Wilshire  Credit  Corporation,  as a servicer  (in such
capacity,  a  "Servicer"),  Ocwen  Federal  Bank  FSB,  as a  servicer  (in such
capacity, a "Servicer") and JPMorgan Chase Bank, as trustee (the "Trustee").

Dated:  March 31, 2003


By:  /s/ Matt Ruppel
    -------------------------------------
Name:  Matt Ruppel
Title: Vice President

                                      -5-
<PAGE>


SUPPLEMENTAL  INFORMATION TO BE FURNISHED WITH REPORTS FILED PURSUANT TO SECTION
15(D) OF THE ACT BY REGISTRANTS WHICH HAVE NOT REGISTERED SECURITIES PURSUANT TO
SECTION 12 OF THE ACT.

The  registrant  has not sent an annual report or proxy material to its security
holders.  The registrant  will not be sending an annual report or proxy material
to its security holders subsequent to the filing of this form.

                                      -6-
<PAGE>


                                 EXHIBIT INDEX

Exhibit     Description

 99.1       Servicers' Annual Statement of Compliance
 99.2       Servicers' Annual Independent Accountant's Report



                                      -7-

<PAGE>

EXHIBIT 99.1 - Servicers' Annual Statement of Compliance

                                     -8-
<PAGE>

Wilshire Credit Corporation
1776 SW Madison Street
Portland, oregon  97205
Telephone:  (503) 223-5600
            (800) 776-0100
Facsimile:  (503) 223-8799


ANNUAL SERVICING CERTIFICATION

In connection with those loans serviced by us we hereby certify that:

     1. All real estate taxes,  special  assessments  and any charges that might
become a lien upon the  property and which come due in the first  calendar  year
have been paid.

     2. All FHA insurance  premiums or private mortgage insurance  premiums,  if
applicable, have been paid and are in full force and effect.

     3. An extended mortgage impairment policy is in force,  adequately insuring
all loans in which Investor Savings has an interest.

     4. All interest and/or monthly payment  adjustments for ARM loans have been
made in accordance with the mortgage terms.

     5. Any exceptions to this  certification  are listed on an attachment along
with an explanation  concerning their  completion.  If there are none listed, it
will be considered that there are no exceptions.

     6. If applicable, all property inspections have been completed according to
the provisions of the servicing agreement.

     7. We have a complete Disaster Recovery Plan in place.


WILSHIRE CREDIT CORPORATION

By:  /s/ Jay Memmott
----------------------------
Jay Memmott, President


Date:  March 22, 2002




MANAGEMENT'S ASSERTION ON MINIMUM SERVICING STANDARDS

     As of and for the year ended December 31, 2001, Wilshire Credit Corporation
has complied in all material respects with the minimum  servicing  standards set
forth  in  the  Mortgage  Banker's   Association  of  America's  Uniform  Single
Attestation  Program  for  Mortgage  Bankers.  As of and for this  same  period,
Wilshire  Credit  Corporation  had in effect a  fidelity  bond in the  amount of
$10,000,000.00 and an errors and omission policy in the amount of $5,000,000.00.


By:  /s/ Jay H. Memmott
-----------------------
Jay H. Memmott
President/CEO
March 22, 2002


By:  /s/ Bruce A. Weinstein
---------------------------
Chief Financial Officer
March 22, 2002

                                      -9-
<PAGE>


OCWEN

OFFIER'S CERTIFICATION ON ANNUAL STATEMENT AS TO COMPLIANCE


March 28, 2002


     Pursuant to the applicable sections of the Servicing Agreements,  I, Ronald
M. Faris as officer of Ocwen Federal Bank FSB (the "Bank") confirm that a review
of the  activities  of the Bank ended  December 31, 2001 and of the  performance
under the Servicing  Agreements has been made under my supervision.  To the best
of my  knowledge,  based  on such  review,  the Bank  has  fulfilled  all of its
obligations under the Servicing Agreements throughout the period.


By:  /s/ Ronald M. Faris
------------------------
Ronald M. Faris
President, Ocwen Federal Bank FSB



MANAGEMENT ASSERTION ON COMPLIANCE WITH USAP

March 29, 2002

     As of and for the year ended December 31, 2001, Ocwen Federal Bank FSB (the
"Bank")  has  complied  in all  material  respects  with the  minimum  servicing
standards set forth in the Mortgage Bankers  Association of America's  ("MBA's")
Uniform Single Attestation Program for Mortgage Bankers ("USAP").

     As of and for this same period,  the Bank had in effect a fidelity  bond in
the amount of  $15,000,000  and an errors and omissions  policy in the amount of
$10,000,000.


By:  /s/ Ronald M. Faris
------------------------
Ronald M. Faris
President

By:  /s/ Scott W. Anderson
--------------------------
Scott W. Anderson
Senior Vice President of Residential Assets

By:  /s/ James D. Campbell
--------------------------
James D. Campbell
Director of Investor Reporting



                                      -10-
<PAGE>


EXHIBIT 99.2 - Servicers' Annual Independent Accountant's Report

                                      -11-
<PAGE>

Deloitte & Touche LLP
111 S.W. Fifth Avenue, Suite 3900
Portland, Oregon 97204-3642
Telephone:  (503) 222-1341
Facsimile:  (503) 224-2172
www.us.deloitte.com


INDEPENDENT ACCOUNTANTS' REPORT

To Wilshire Credit Corporation:

     We have examined management's assertion about Wilshire Credit Corporation's
(the "Company"),  a  majority-owned  subsidiary of Wilshire  Financial  Services
Group, Inc.,  compliance with the minimum servicing standards  identified in the
Mortgage Bankers Association of America's Uniform Single Attestation Program for
Mortgagae  Bankers  (USAP)  as of and for the  year  ended  December  31,  2001,
included in the accompanying management assertion. Management is responsible for
the  Company's   compliance  with  those  minimum   servicing   standards.   Our
responsibility  is to express an opinion  on  management's  assertion  about the
entity's compliance based on our examination.

     Our examination  was made in accordance  with standards  established by the
American  Institute of Certified Public Accountants and,  accordingly,  included
examining,  on a test basis,  evidence about the Company's  compliance  with the
minimum  servicing   standards  and  performing  such  other  procedures  as  we
considered  necessary  in the  circumstances.  We believe  that our  examination
provides a reasonable basis for our opinion.  Our examination does not provide a
legal  determination  on the  Company's  compliance  with the minimum  servicing
standards.

     In our opinion,  management's  assertion that the Company complied with the
aforementioned minimum servicing standards as of and for the year ended December
31, 2001 is fairly stated, in all material respects.


By:  /s/ Deloitte & Touche, LLP
--------------------------------
Deloitte & Touche, LLP


March 22, 2002

                                      -11-

<PAGE>

PRICEWATERHOUSECOOPERS LLP
222 Lakeview Avenue, Suite 360
West Palm Beach, FL  33401
Telephone:  (561) 832-0038
Facsimile:  (561) 805-8181


INDEPENDENT CERTIFIED PUBLIC ACCOUNTANT'S REPORT

To the Board of Directors of
Ocwen Federal Bank FSB

     We have  examined  management's  assertion,  included  in the  accompanying
Management  Assertion on Compliance  with USAP, that Ocwen Federal Bank FSB (the
"Bank") complied with the minimum servicing standards identified in the Mortgage
Bankers   Association  of  America's  Uniform  Single  Attestation  Program  for
Mortgagae  Bankers  ("USAP")  as of and for the year ended  December  31,  2001.
Management is responsible for the Bank's compliance with those minimum servicing
standards. Our responsibility is to express an opinion on management's assertion
about the Bank's compliance based on our examination.

     Our  examination  was  made  in  accordance  with   attestation   standards
established  by the  American  Institute of Certified  Public  Accountants  and,
accordingly,  included  examining,  on a test basis,  evidence  about the Bank's
compliance  with the  minimum  servicing  standards  and  performing  such other
procedures as we considered necessary in the circumstances.  We believe that our
examination  provides a reasonable  basis for our opinion.  Our examination does
not  provide a legal  determination  on the Bank's  compliance  with the minimum
servicing standards.

     In our opinion,  management's  assertion  that the Bank  complied  with the
aforementioned minimum servicing standards as of and for the year ended December
31, 2001 is fairly stated, in all material respects.


By:  /s/  PricewaterhouseCoopers, LLP
-------------------------------------
PricewaterhouseCoopers, LLP
West Palm Beach, Florida
March 29, 2002

                                      -12-