MORGAN STANLEY S&P 500 INDEX FUND
                           1221 Avenue of the Americas
                            New York, New York 10020




                                                               December 20, 2006



Securities and Exchange Commission
100 F Street, N.E.
Washington, D.C. 20549

Re:      File Nos.:  333-29721; 811-8265
         Post-Effective Amendment No. 15
         -------------------------------


Dear Sir or Madam:

         We are electronically filing via EDGAR, pursuant to the provisions of
the Investment Company Act of 1940 and the Securities Act of 1933 and Rule
485(b) thereunder a copy of Post-Effective Amendment No. 15 to the Registration
Statement of this Fund on Form N-1A.

         The Registration Statement is marked to reflect all changes from the
Prospectus and Statement of Additional Information filed pursuant to Rule 485(a)
on October 26, 2006.

         Also enclosed, pursuant to Rule 485(b), is the required representation
of Counsel that the filing does not contain any disclosures that would render it
ineligible to become effective under the Rule.



                                              Very truly yours,

                                              /s/ Eric C. Griffith
                                              --------------------
                                              Eric C. Griffith
                                              Assistant Secretary



Enclosures
cc:  Amy R. Doberman, Esq.