<PAGE> PAGE 1
000 A000000 02/28/98
000 C000000 0001041237
000 D000000 N
000 E000000 NF
000 F000000 Y
000 G000000 N
000 H000000 N
000 I000000 3.0.a
000 J000000 A
001 A000000 DEAN WITTER S&P 500 INDEX FUND
001 B000000 811-08265
001 C000000 2123921600
002 A000000 2 WORLD TRADE CENTER
002 B000000 NEW YORK
002 C000000 NY
002 D010000 10048
003 000000 Y
004 000000 N
005 000000 N
006 000000 N
007 A000000 N
007 B000000 0
007 C010100 1
007 C010200 2
007 C010300 3
007 C010400 4
007 C010500 5
007 C010600 6
007 C010700 7
007 C010800 8
007 C010900 9
007 C011000 10
008 A000001 DEAN WITTER INTERCAPITAL INC.
008 B000001 A
008 C000001 801-42061
008 D010001 NEW YORK
008 D020001 NY
008 D030001 10048
011 A000001 DEAN WITTER DISTRIBUTORS INC.
011 B000001 8-45262
011 C010001 NEW YORK
011 C020001 NY
011 C030001 10048
012 A000001 MORGAN STANLEY DEAN WITTER TRUST FSB
012 B000001 84-1785
012 C010001 JERSEY CITY
012 C020001 NJ
012 C030001 07311
013 A000001 PRICE WATERHOUSE LLP
013 B010001 NEW YORK
013 B020001 NY
<PAGE> PAGE 2
013 B030001 10036
014 A000001 DEAN WITTER REYNOLDS INC.
014 B000001 8-14172
014 A000002 MORGAN STANLEY & CO. INCORPORATED
014 B000002 8-15869
015 A000001 THE BANK OF NEW YORK
015 B000001 C
015 C010001 NEW YORK
015 C020001 NY
015 C030001 10286
015 E010001 X
018 000000 Y
019 A000000 Y
019 B000000 130
019 C000000 DEANWITTER
020 A000001 CANTOR FITZGERALD
020 B000001 95-1786286
020 C000001 45
020 A000002 NOMURA SECURITIES INTERNATIONAL INC.
020 B000002 13-2642206
020 C000002 15
020 A000003 SMITH BARNEY INC.
020 B000003 13-1912900
020 C000003 2
020 C000004 0
020 C000005 0
020 C000006 0
020 C000007 0
020 C000008 0
020 C000009 0
020 C000010 0
021 000000 62
022 A000001 GOLDMAN SACHS & CO.
022 B000001 13-5108880
022 C000001 569988
022 D000001 0
022 A000002 BANK OF NEW YORK (THE)
022 B000002 13-5160382
022 C000002 370218
022 D000002 0
022 A000003 BANCAMERICA ROBERTSON STEPHENS
022 B000003 95-4035346
022 C000003 350130
022 D000003 0
022 A000004 NOMURA SECURITIES INTERNATIONAL INC.
022 B000004 13-2642206
022 C000004 208050
022 D000004 19
022 A000005 CHASE SECURITIES INC.
022 B000005 13-3112953
022 C000005 187914
<PAGE> PAGE 3
022 D000005 0
022 A000006 MERRILL LYNCH PIERCE FENNER & SMITH INC.
022 B000006 13-5674085
022 C000006 56281
022 D000006 0
022 A000007 CS FIRST BOSTON CORPORATION
022 B000007 13-5659485
022 C000007 29136
022 D000007 4499
022 A000008 CANTOR FITZGERALD
022 B000008 95-1786286
022 C000008 16618
022 D000008 0
022 A000009 FIRST CHICAGO CAPITAL MARKETS INC.
022 B000009 36-3595942
022 C000009 8992
022 D000009 0
022 A000010 MORGAN (J.P.) SECURITIES INC.
022 B000010 13-3224016
022 C000010 6993
022 D000010 0
023 C000000 1805313
023 D000000 4519
024 000000 N
025 A000001 DELETE
025 D000001 0
025 A000002 DELETE
025 D000002 0
025 D000003 0
025 D000004 0
025 D000005 0
025 D000006 0
025 D000007 0
025 D000008 0
025 D000009 0
025 D000010 0
025 D000011 0
025 D000012 0
025 D000013 0
025 D000014 0
025 D000015 0
025 D000016 0
026 A000000 N
026 B000000 Y
026 C000000 N
026 D000000 Y
026 E000000 Y
026 F000000 N
026 G010000 N
026 G020000 N
026 H000000 N
<PAGE> PAGE 4
027 000000 Y
028 A010000 182981
028 A020000 0
028 A030000 0
028 A040000 0
028 B010000 59400
028 B020000 0
028 B030000 0
028 B040000 1621
028 C010000 41259
028 C020000 0
028 C030000 0
028 C040000 2091
028 D010000 53086
028 D020000 245
028 D030000 0
028 D040000 13328
028 E010000 41989
028 E020000 0
028 E030000 0
028 E040000 8500
028 F010000 53160
028 F020000 0
028 F030000 0
028 F040000 8873
028 G010000 431875
028 G020000 245
028 G030000 0
028 G040000 34413
028 H000000 431875
029 000000 Y
030 A000000 318
030 B000000 0.00
030 C000000 0.00
031 A000000 0
031 B000000 0
032 000000 0
033 000000 318
034 000000 Y
035 000000 180
036 A000000 Y
036 B000000 0
037 000000 N
038 000000 0
039 000000 N
040 000000 Y
041 000000 Y
042 A000000 0
042 B000000 0
042 C000000 100
042 D000000 0
<PAGE> PAGE 5
042 E000000 0
042 F000000 0
042 G000000 0
042 H000000 0
043 000000 1216
044 000000 0
045 000000 Y
046 000000 N
047 000000 Y
048 000000 0.400
048 A010000 0
048 A020000 0.000
048 B010000 0
048 B020000 0.000
048 C010000 0
048 C020000 0.000
048 D010000 0
048 D020000 0.000
048 E010000 0
048 E020000 0.000
048 F010000 0
048 F020000 0.000
048 G010000 0
048 G020000 0.000
048 H010000 0
048 H020000 0.000
048 I010000 0
048 I020000 0.000
048 J010000 0
048 J020000 0.000
048 K010000 0
048 K020000 0.000
049 000000 N
050 000000 N
051 000000 N
052 000000 N
053 A000000 N
054 A000000 Y
054 B000000 Y
054 C000000 Y
054 D000000 N
054 E000000 N
054 F000000 N
054 G000000 N
054 H000000 Y
054 I000000 N
054 J000000 Y
054 K000000 N
054 L000000 Y
054 M000000 Y
054 N000000 N
<PAGE> PAGE 6
054 O000000 Y
055 A000000 N
055 B000000 N
056 000000 Y
057 000000 N
058 A000000 N
059 000000 Y
060 A000000 Y
060 B000000 Y
061 000000 1000
062 A000000 N
062 B000000 0.0
062 C000000 0.0
062 D000000 0.0
062 E000000 0.0
062 F000000 0.0
062 G000000 0.0
062 H000000 0.0
062 I000000 0.0
062 J000000 0.0
062 K000000 0.0
062 L000000 0.0
062 M000000 0.0
062 N000000 0.0
062 O000000 0.0
062 P000000 0.0
062 Q000000 0.0
062 R000000 0.0
063 A000000 0
063 B000000 0.0
066 A000000 Y
066 B000000 N
066 C000000 N
066 D000000 N
066 E000000 N
066 F000000 N
066 G000000 Y
067 000000 N
068 A000000 N
068 B000000 N
069 000000 Y
070 A010000 Y
070 A020000 Y
070 B010000 N
070 B020000 N
070 C010000 N
070 C020000 N
070 D010000 N
070 D020000 N
070 E010000 N
070 E020000 N
<PAGE> PAGE 7
070 F010000 Y
070 F020000 Y
070 G010000 N
070 G020000 N
070 H010000 N
070 H020000 N
070 I010000 N
070 I020000 N
070 J010000 N
070 J020000 N
070 K010000 Y
070 K020000 N
070 L010000 Y
070 L020000 Y
070 M010000 N
070 M020000 N
070 N010000 N
070 N020000 N
070 O010000 N
070 O020000 N
070 P010000 N
070 P020000 N
070 Q010000 N
070 Q020000 N
070 R010000 N
070 R020000 N
071 A000000 391999
071 B000000 1791
071 C000000 296396
071 D000000 1
072 A000000 6
072 B000000 399
072 C000000 1975
072 D000000 0
072 E000000 0
072 F000000 510
072 G000000 0
072 H000000 0
072 I000000 118
072 J000000 38
072 K000000 0
072 L000000 6
072 M000000 6
072 N000000 137
072 O000000 0
072 P000000 0
072 Q000000 0
072 R000000 10
072 S000000 8
072 T000000 1216
072 U000000 4
<PAGE> PAGE 8
072 V000000 13
072 W000000 0
072 X000000 2066
072 Y000000 212
072 Z000000 520
072AA000000 0
072BB000000 201
072CC010000 38103
072CC020000 0
072DD010000 251
072DD020000 49
072EE000000 0
073 A010000 0.0000
073 A020000 0.0000
073 B000000 0.0000
073 C000000 0.0000
074 A000000 26
074 B000000 2915
074 C000000 0
074 D000000 0
074 E000000 0
074 F000000 428106
074 G000000 0
074 H000000 0
074 I000000 0
074 J000000 0
074 K000000 0
074 L000000 6268
074 M000000 46
074 N000000 437361
074 O000000 0
074 P000000 358
074 Q000000 0
074 R010000 0
074 R020000 0
074 R030000 0
074 R040000 1074
074 S000000 0
074 T000000 435929
074 U010000 35951
074 U020000 3315
074 V010000 0.00
074 V020000 0.00
074 W000000 0.0000
074 X000000 34127
074 Y000000 0
075 A000000 0
075 B000000 298526
076 000000 0.00
077 A000000 N
078 000000 N
<PAGE> PAGE 9
080 C000000 0
081 B000000 0
082 B000000 0
083 B000000 0
084 B000000 0
086 A010000 0
086 A020000 0
086 B010000 0
086 B020000 0
086 C010000 0
086 C020000 0
086 D010000 0
086 D020000 0
086 E010000 0
086 E020000 0
086 F010000 0
086 F020000 0
SIGNATURE BARRY FINK
TITLE VICE PRESIDENT
WARNING: THE EDGAR SYSTEM ENCOUNTERED ERROR(S) WHILE PROCESSING THIS SCHEDULE.
<TABLE> <S> <C>
<ARTICLE> 6
<LEGEND>
Dean Witter S&P 500 Index Fund
Semi-Annual NSAR 2/28/98 Class A Shares
</LEGEND>
<CIK> 0001041237
<NAME> DW S&P 500 INDEX FUND
<S> <C>
<PERIOD-TYPE> 6-MOS
<FISCAL-YEAR-END> AUG-31-1998
<PERIOD-END> FEB-28-1998
<INVESTMENTS-AT-COST> 393,056,007
<INVESTMENTS-AT-VALUE> 431,020,945
<RECEIVABLES> 6,268,238
<ASSETS-OTHER> 46,251
<OTHER-ITEMS-ASSETS> 25,567
<TOTAL-ASSETS> 437,361,001
<PAYABLE-FOR-SECURITIES> 0
<SENIOR-LONG-TERM-DEBT> 0
<OTHER-ITEMS-LIABILITIES> 1,432,021
<TOTAL-LIABILITIES> 1,432,021
<SENIOR-EQUITY> 0
<PAID-IN-CAPITAL-COMMON> 397,807,150
<SHARES-COMMON-STOCK> 1,716,869
<SHARES-COMMON-PRIOR> 0
<ACCUMULATED-NII-CURRENT> 219,833
<OVERDISTRIBUTION-NII> 0
<ACCUMULATED-NET-GAINS> (201,209)
<OVERDISTRIBUTION-GAINS> 0
<ACCUM-APPREC-OR-DEPREC> 38,103,206
<NET-ASSETS> 19,088,917
<DIVIDEND-INCOME> 1,975,191
<INTEREST-INCOME> 398,787
<OTHER-INCOME> 0
<EXPENSES-NET> 1,854,124
<NET-INVESTMENT-INCOME> 519,854
<REALIZED-GAINS-CURRENT> (201,209)
<APPREC-INCREASE-CURRENT> 38,103,206
<NET-CHANGE-FROM-OPS> 38,421,851
<EQUALIZATION> 0
<DISTRIBUTIONS-OF-INCOME> (32,194)
<DISTRIBUTIONS-OF-GAINS> 0
<DISTRIBUTIONS-OTHER> 0
<NUMBER-OF-SHARES-SOLD> 2,174,952
<NUMBER-OF-SHARES-REDEEMED> (463,575)
<SHARES-REINVESTED> 2,992
<NET-CHANGE-IN-ASSETS> 435,828,980
<ACCUMULATED-NII-PRIOR> 0
<ACCUMULATED-GAINS-PRIOR> 0
<OVERDISTRIB-NII-PRIOR> 0
<OVERDIST-NET-GAINS-PRIOR> 0
<GROSS-ADVISORY-FEES> 510,357
<INTEREST-EXPENSE> 0
<GROSS-EXPENSE> 2,066,376
<AVERAGE-NET-ASSETS> 12,365,913
<PER-SHARE-NAV-BEGIN> 10.00
<PER-SHARE-NII> 0.05
<PER-SHARE-GAIN-APPREC> 1.10
<PER-SHARE-DIVIDEND> (0.03)
<PER-SHARE-DISTRIBUTIONS> 0
<RETURNS-OF-CAPITAL> 0
<PER-SHARE-NAV-END> 11.12
<EXPENSE-RATIO> 0.74
<AVG-DEBT-OUTSTANDING> 0
<AVG-DEBT-PER-SHARE> 0
</TABLE>
WARNING: THE EDGAR SYSTEM ENCOUNTERED ERROR(S) WHILE PROCESSING THIS SCHEDULE.
<TABLE> <S> <C>
<ARTICLE> 6
<LEGEND>
Dean Witter S&P 500 Index Fund
Semi-Annual NSAR 2/28/98 Class B Shares
</LEGEND>
<CIK> 0001041237
<NAME> DW S&P 500 INDEX FUND
<S> <C>
<PERIOD-TYPE> 6-MOS
<FISCAL-YEAR-END> AUG-31-1998
<PERIOD-END> FEB-28-1998
<INVESTMENTS-AT-COST> 393,056,007
<INVESTMENTS-AT-VALUE> 431,020,945
<RECEIVABLES> 6,268,238
<ASSETS-OTHER> 46,251
<OTHER-ITEMS-ASSETS> 25,567
<TOTAL-ASSETS> 437,361,001
<PAYABLE-FOR-SECURITIES> 0
<SENIOR-LONG-TERM-DEBT> 0
<OTHER-ITEMS-LIABILITIES> 1,432,021
<TOTAL-LIABILITIES> 1,432,021
<SENIOR-EQUITY> 0
<PAID-IN-CAPITAL-COMMON> 397,807,150
<SHARES-COMMON-STOCK> 34,233,805
<SHARES-COMMON-PRIOR> 0
<ACCUMULATED-NII-CURRENT> 219,833
<OVERDISTRIBUTION-NII> 0
<ACCUMULATED-NET-GAINS> (201,209)
<OVERDISTRIBUTION-GAINS> 0
<ACCUM-APPREC-OR-DEPREC> 38,103,206
<NET-ASSETS> 380,009,145
<DIVIDEND-INCOME> 1,975,191
<INTEREST-INCOME> 398,787
<OTHER-INCOME> 0
<EXPENSES-NET> 1,854,124
<NET-INVESTMENT-INCOME> 519,854
<REALIZED-GAINS-CURRENT> (201,209)
<APPREC-INCREASE-CURRENT> 38,103,206
<NET-CHANGE-FROM-OPS> 38,421,851
<EQUALIZATION> 0
<DISTRIBUTIONS-OF-INCOME> (218,935)
<DISTRIBUTIONS-OF-GAINS> 0
<DISTRIBUTIONS-OTHER> 0
<NUMBER-OF-SHARES-SOLD> 35,877,610
<NUMBER-OF-SHARES-REDEEMED> (1,665,955)
<SHARES-REINVESTED> 19,650
<NET-CHANGE-IN-ASSETS> 435,828,980
<ACCUMULATED-NII-PRIOR> 0
<ACCUMULATED-GAINS-PRIOR> 0
<OVERDISTRIB-NII-PRIOR> 0
<OVERDIST-NET-GAINS-PRIOR> 0
<GROSS-ADVISORY-FEES> 510,357
<INTEREST-EXPENSE> 0
<GROSS-EXPENSE> 2,066,376
<AVERAGE-NET-ASSETS> 263,834,128
<PER-SHARE-NAV-BEGIN> 10.00
<PER-SHARE-NII> 0.02
<PER-SHARE-GAIN-APPREC> 1.09
<PER-SHARE-DIVIDEND> (0.01)
<PER-SHARE-DISTRIBUTIONS> 0
<RETURNS-OF-CAPITAL> 0
<PER-SHARE-NAV-END> 11.10
<EXPENSE-RATIO> 1.50
<AVG-DEBT-OUTSTANDING> 0
<AVG-DEBT-PER-SHARE> 0
</TABLE>
WARNING: THE EDGAR SYSTEM ENCOUNTERED ERROR(S) WHILE PROCESSING THIS SCHEDULE.
<TABLE> <S> <C>
<ARTICLE> 6
<LEGEND>
Dean Witter S&P 500 Index Fund
Semi-Annual NSAR 2/28/98 Class C Shares
</LEGEND>
<CIK> 0001041237
<NAME> DW S&P 500 INDEX FUND
<S> <C>
<PERIOD-TYPE> 6-MOS
<FISCAL-YEAR-END> AUG-31-1998
<PERIOD-END> FEB-28-1998
<INVESTMENTS-AT-COST> 393,056,007
<INVESTMENTS-AT-VALUE> 431,020,945
<RECEIVABLES> 6,268,238
<ASSETS-OTHER> 46,251
<OTHER-ITEMS-ASSETS> 25,567
<TOTAL-ASSETS> 437,361,001
<PAYABLE-FOR-SECURITIES> 0
<SENIOR-LONG-TERM-DEBT> 0
<OTHER-ITEMS-LIABILITIES> 1,432,021
<TOTAL-LIABILITIES> 1,432,021
<SENIOR-EQUITY> 0
<PAID-IN-CAPITAL-COMMON> 397,807,150
<SHARES-COMMON-STOCK> 2,173,252
<SHARES-COMMON-PRIOR> 0
<ACCUMULATED-NII-CURRENT> 219,833
<OVERDISTRIBUTION-NII> 0
<ACCUMULATED-NET-GAINS> (201,209)
<OVERDISTRIBUTION-GAINS> 0
<ACCUM-APPREC-OR-DEPREC> 38,103,206
<NET-ASSETS> 24,126,224
<DIVIDEND-INCOME> 1,975,191
<INTEREST-INCOME> 398,787
<OTHER-INCOME> 0
<EXPENSES-NET> 1,854,124
<NET-INVESTMENT-INCOME> 519,854
<REALIZED-GAINS-CURRENT> (201,209)
<APPREC-INCREASE-CURRENT> 38,103,206
<NET-CHANGE-FROM-OPS> 38,421,851
<EQUALIZATION> 0
<DISTRIBUTIONS-OF-INCOME> (12,314)
<DISTRIBUTIONS-OF-GAINS> 0
<DISTRIBUTIONS-OTHER> 0
<NUMBER-OF-SHARES-SOLD> 2,361,111
<NUMBER-OF-SHARES-REDEEMED> (191,494)
<SHARES-REINVESTED> 1,135
<NET-CHANGE-IN-ASSETS> 435,828,980
<ACCUMULATED-NII-PRIOR> 0
<ACCUMULATED-GAINS-PRIOR> 0
<OVERDISTRIB-NII-PRIOR> 0
<OVERDIST-NET-GAINS-PRIOR> 0
<GROSS-ADVISORY-FEES> 510,357
<INTEREST-EXPENSE> 0
<GROSS-EXPENSE> 2,066,376
<AVERAGE-NET-ASSETS> 17,713,952
<PER-SHARE-NAV-BEGIN> 10.00
<PER-SHARE-NII> 0.02
<PER-SHARE-GAIN-APPREC> 1.09
<PER-SHARE-DIVIDEND> (0.01)
<PER-SHARE-DISTRIBUTIONS> 0
<RETURNS-OF-CAPITAL> 0
<PER-SHARE-NAV-END> 11.10
<EXPENSE-RATIO> 1.50
<AVG-DEBT-OUTSTANDING> 0
<AVG-DEBT-PER-SHARE> 0
</TABLE>
<TABLE> <S> <C>
<ARTICLE> 6
<LEGEND>
Dean Witter S&P 500 Index Fund
Semi-Annual NSAR 2/28/98 Class D Shares
</LEGEND>
<CIK> 0001041237
<NAME> DW S&P 500 INDEX FUND
<S> <C>
<PERIOD-TYPE> 6-MOS
<FISCAL-YEAR-END> AUG-31-1998
<PERIOD-END> FEB-28-1998
<INVESTMENTS-AT-COST> 393,056,007
<INVESTMENTS-AT-VALUE> 431,020,945
<RECEIVABLES> 6,268,238
<ASSETS-OTHER> 46,251
<OTHER-ITEMS-ASSETS> 25,567
<TOTAL-ASSETS> 437,361,001
<PAYABLE-FOR-SECURITIES> 0
<SENIOR-LONG-TERM-DEBT> 0
<OTHER-ITEMS-LIABILITIES> 1,432,021
<TOTAL-LIABILITIES> 1,432,021
<SENIOR-EQUITY> 0
<PAID-IN-CAPITAL-COMMON> 397,807,150
<SHARES-COMMON-STOCK> 1,142,033
<SHARES-COMMON-PRIOR> 0
<ACCUMULATED-NII-CURRENT> 219,833
<OVERDISTRIBUTION-NII> 0
<ACCUMULATED-NET-GAINS> (201,209)
<OVERDISTRIBUTION-GAINS> 0
<ACCUM-APPREC-OR-DEPREC> 38,103,206
<NET-ASSETS> 12,704,694
<DIVIDEND-INCOME> 1,975,191
<INTEREST-INCOME> 398,787
<OTHER-INCOME> 0
<EXPENSES-NET> 1,854,124
<NET-INVESTMENT-INCOME> 519,854
<REALIZED-GAINS-CURRENT> (201,209)
<APPREC-INCREASE-CURRENT> 38,103,206
<NET-CHANGE-FROM-OPS> 38,421,851
<EQUALIZATION> 0
<DISTRIBUTIONS-OF-INCOME> (36,578)
<DISTRIBUTIONS-OF-GAINS> 0
<DISTRIBUTIONS-OTHER> 0
<NUMBER-OF-SHARES-SOLD> 2,170,273
<NUMBER-OF-SHARES-REDEEMED> (1,030,929)
<SHARES-REINVESTED> 189
<NET-CHANGE-IN-ASSETS> 435,828,980
<ACCUMULATED-NII-PRIOR> 0
<ACCUMULATED-GAINS-PRIOR> 0
<OVERDISTRIB-NII-PRIOR> 0
<OVERDIST-NET-GAINS-PRIOR> 0
<GROSS-ADVISORY-FEES> 510,357
<INTEREST-EXPENSE> 0
<GROSS-EXPENSE> 2,066,376
<AVERAGE-NET-ASSETS> 4,612,306
<PER-SHARE-NAV-BEGIN> 10.00
<PER-SHARE-NII> 0.06
<PER-SHARE-GAIN-APPREC> 1.09
<PER-SHARE-DIVIDEND> (0.03)
<PER-SHARE-DISTRIBUTIONS> 0
<RETURNS-OF-CAPITAL> 0
<PER-SHARE-NAV-END> 11.12
<EXPENSE-RATIO> 0.50
<AVG-DEBT-OUTSTANDING> 0
<AVG-DEBT-PER-SHARE> 0
</TABLE>
