<Page> Exhibit 99.906CERT SECTION 906 CERTIFICATION Certification Pursuant to 18 U.S.C. Section 1350, As Adopted Pursuant to Section 906 of the Sarbanes-Oxley Act of 2002 Morgan Stanley S&P 500 Index Fund In connection with the Report on Form N-CSR (the "Report") of the above-named issuer for the period ended February 28, 2006 that is accompanied by this certification, the undersigned hereby certifies that: 1. The Report fully complies with the requirements of Section 13(a) or 15(d) of the Securities Exchange Act of 1934; and 2. The information contained in the Report fairly presents, in all material respects, the financial condition and results of operations of the Issuer. Date: April 19, 2006 /s/ Ronald E. Robison --------------------- Ronald E. Robison Principal Executive Officer A signed original of this written statement required by Section 906 has been provided to Morgan Stanley S&P 500 Index Fund and will be retained by Morgan Stanley S&P 500 Index Fund and furnished to the Securities and Exchange Commission or its staff upon request. <Page> SECTION 906 CERTIFICATION Certification Pursuant to 18 U.S.C. Section 1350, As Adopted Pursuant to Section 906 of the Sarbanes-Oxley Act of 2002 Morgan Stanley S&P 500 Index Fund In connection with the Report on Form N-CSR (the "Report") of the above-named issuer for the period ended February 28, 2006 that is accompanied by this certification, the undersigned hereby certifies that: 1. The Report fully complies with the requirements of Section 13(a) or 15(d) of the Securities Exchange Act of 1934; and 2. The information contained in the Report fairly presents, in all material respects, the financial condition and results of operations of the Issuer. Date: April 19, 2006 /s/ Francis Smith ----------------- Francis Smith Principal Financial Officer A signed original of this written statement required by Section 906 has been provided to Morgan Stanley S&P 500 Index Fund and will be retained by Morgan Stanley S&P 500 Index Fund and furnished to the Securities and Exchange Commission or its staff upon request.