<PAGE> PAGE 1
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<PAGE> PAGE 2
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<PAGE> PAGE 3
020 C000010 45
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<PAGE> PAGE 4
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<PAGE> PAGE 5
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<PAGE> PAGE 6
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<PAGE> PAGE 8
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<PAGE> PAGE 10
SIGNATURE ROBERT P. NAULT
TITLE ASST. SECRETARY
WARNING: THE EDGAR SYSTEM ENCOUNTERED ERROR(S) WHILE PROCESSING THIS SCHEDULE.
<TABLE> <S> <C>
<ARTICLE> 6
<CIK> 0000706155
<NAME> PIONEER MID-CAP FUND
<SERIES>
<NUMBER> 001
<NAME> PIONEER MID-CAP FUND CLASS A
<S> <C>
<PERIOD-TYPE> 6-MOS
<FISCAL-YEAR-END> SEP-30-1999
<PERIOD-END> MAR-31-1999
<INVESTMENTS-AT-COST> 697941153
<INVESTMENTS-AT-VALUE> 825094225
<RECEIVABLES> 10935903
<ASSETS-OTHER> 23015
<OTHER-ITEMS-ASSETS> 742
<TOTAL-ASSETS> 836053885
<PAYABLE-FOR-SECURITIES> 78400
<SENIOR-LONG-TERM-DEBT> 0
<OTHER-ITEMS-LIABILITIES> 3932290
<TOTAL-LIABILITIES> 4010690
<SENIOR-EQUITY> 0
<PAID-IN-CAPITAL-COMMON> 593681873
<SHARES-COMMON-STOCK> 45587684
<SHARES-COMMON-PRIOR> 46411713
<ACCUMULATED-NII-CURRENT> 0
<OVERDISTRIBUTION-NII> 1231671
<ACCUMULATED-NET-GAINS> 112439921
<OVERDISTRIBUTION-GAINS> 0
<ACCUM-APPREC-OR-DEPREC> 127153072
<NET-ASSETS> 832043195
<DIVIDEND-INCOME> 2087846
<INTEREST-INCOME> 395079
<OTHER-INCOME> 0
<EXPENSES-NET> 3714596
<NET-INVESTMENT-INCOME> (1231671)
<REALIZED-GAINS-CURRENT> 116497254
<APPREC-INCREASE-CURRENT> 24867151
<NET-CHANGE-FROM-OPS> 140132734
<EQUALIZATION> 0
<DISTRIBUTIONS-OF-INCOME> 0
<DISTRIBUTIONS-OF-GAINS> 66015386
<DISTRIBUTIONS-OTHER> 0
<NUMBER-OF-SHARES-SOLD> 5902982
<NUMBER-OF-SHARES-REDEEMED> 10518466
<SHARES-REINVESTED> 3791455
<NET-CHANGE-IN-ASSETS> 57357119
<ACCUMULATED-NII-PRIOR> 0
<ACCUMULATED-GAINS-PRIOR> 63002076
<OVERDISTRIB-NII-PRIOR> 0
<OVERDIST-NET-GAINS-PRIOR> 0
<GROSS-ADVISORY-FEES> 1762853
<INTEREST-EXPENSE> 4702
<GROSS-EXPENSE> 3773898
<AVERAGE-NET-ASSETS> 820053865
<PER-SHARE-NAV-BEGIN> 16.53
<PER-SHARE-NII> (0.03)
<PER-SHARE-GAIN-APPREC> 2.93
<PER-SHARE-DIVIDEND> 0
<PER-SHARE-DISTRIBUTIONS> 1.45
<RETURNS-OF-CAPITAL> 0
<PER-SHARE-NAV-END> 17.98
<EXPENSE-RATIO> 0.90
[AVG-DEBT-OUTSTANDING] 163352
[AVG-DEBT-PER-SHARE] 0.003
</TABLE>
WARNING: THE EDGAR SYSTEM ENCOUNTERED ERROR(S) WHILE PROCESSING THIS SCHEDULE.
<TABLE> <S> <C>
<ARTICLE> 6
<CIK> 0000706155
<NAME> PIONEER MID-CAP FUND
<SERIES>
<NUMBER> 002
<NAME> PIONEER MID-CAP FUND CLASS B
<S> <C>
<PERIOD-TYPE> 6-MOS
<FISCAL-YEAR-END> SEP-30-1999
<PERIOD-END> MAR-31-1999
<INVESTMENTS-AT-COST> 697941153
<INVESTMENTS-AT-VALUE> 825094225
<RECEIVABLES> 10935903
<ASSETS-OTHER> 23015
<OTHER-ITEMS-ASSETS> 742
<TOTAL-ASSETS> 836053885
<PAYABLE-FOR-SECURITIES> 78400
<SENIOR-LONG-TERM-DEBT> 0
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<TOTAL-LIABILITIES> 4010690
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<DIVIDEND-INCOME> 2087846
<INTEREST-INCOME> 395079
<OTHER-INCOME> 0
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<REALIZED-GAINS-CURRENT> 116497254
<APPREC-INCREASE-CURRENT> 24867151
<NET-CHANGE-FROM-OPS> 140132734
<EQUALIZATION> 0
<DISTRIBUTIONS-OF-INCOME> 0
<DISTRIBUTIONS-OF-GAINS> 845625
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<SHARES-REINVESTED> 49030
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<ACCUMULATED-NII-PRIOR> 0
<ACCUMULATED-GAINS-PRIOR> 63002076
<OVERDISTRIB-NII-PRIOR> 0
<OVERDIST-NET-GAINS-PRIOR> 0
<GROSS-ADVISORY-FEES> 1762853
<INTEREST-EXPENSE> 4702
<GROSS-EXPENSE> 3773898
<AVERAGE-NET-ASSETS> 9391601
<PER-SHARE-NAV-BEGIN> 15.99
<PER-SHARE-NII> (0.11)
<PER-SHARE-GAIN-APPREC> 2.82
<PER-SHARE-DIVIDEND> 0
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<RETURNS-OF-CAPITAL> 0
<PER-SHARE-NAV-END> 17.25
<EXPENSE-RATIO> 1.92
[AVG-DEBT-OUTSTANDING] 163352
[AVG-DEBT-PER-SHARE] 0.003
</TABLE>
WARNING: THE EDGAR SYSTEM ENCOUNTERED ERROR(S) WHILE PROCESSING THIS SCHEDULE.
<TABLE> <S> <C>
<ARTICLE> 6
<CIK> 0000706155
<NAME> PIONEER MID-CAP FUND
<SERIES>
<NUMBER> 003
<NAME> PIONEER MID-CAP FUND CLASS C
<S> <C>
<PERIOD-TYPE> 6-MOS
<FISCAL-YEAR-END> SEP-30-1999
<PERIOD-END> MAR-31-1999
<INVESTMENTS-AT-COST> 697941153
<INVESTMENTS-AT-VALUE> 825094225
<RECEIVABLES> 10935903
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<OTHER-ITEMS-ASSETS> 742
<TOTAL-ASSETS> 836053885
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<SENIOR-LONG-TERM-DEBT> 0
<OTHER-ITEMS-LIABILITIES> 3932290
<TOTAL-LIABILITIES> 4010690
<SENIOR-EQUITY> 0
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<SHARES-COMMON-STOCK> 157853
<SHARES-COMMON-PRIOR> 89580
<ACCUMULATED-NII-CURRENT> 0
<OVERDISTRIBUTION-NII> 1231671
<ACCUMULATED-NET-GAINS> 112439921
<OVERDISTRIBUTION-GAINS> 0
<ACCUM-APPREC-OR-DEPREC> 127153072
<NET-ASSETS> 832043195
<DIVIDEND-INCOME> 2087846
<INTEREST-INCOME> 395079
<OTHER-INCOME> 0
<EXPENSES-NET> 3714596
<NET-INVESTMENT-INCOME> (1231671)
<REALIZED-GAINS-CURRENT> 116497254
<APPREC-INCREASE-CURRENT> 24867151
<NET-CHANGE-FROM-OPS> 140132734
<EQUALIZATION> 0
<DISTRIBUTIONS-OF-INCOME> 0
<DISTRIBUTIONS-OF-GAINS> 198398
<DISTRIBUTIONS-OTHER> 0
<NUMBER-OF-SHARES-SOLD> 613240
<NUMBER-OF-SHARES-REDEEMED> 555700
<SHARES-REINVESTED> 10733
<NET-CHANGE-IN-ASSETS> 57357119
<ACCUMULATED-NII-PRIOR> 0
<ACCUMULATED-GAINS-PRIOR> 63002076
<OVERDISTRIB-NII-PRIOR> 0
<OVERDIST-NET-GAINS-PRIOR> 0
<GROSS-ADVISORY-FEES> 1762853
<INTEREST-EXPENSE> 4702
<GROSS-EXPENSE> 3773898
<AVERAGE-NET-ASSETS> 2411377
<PER-SHARE-NAV-BEGIN> 16.30
<PER-SHARE-NII> (0.11)
<PER-SHARE-GAIN-APPREC> 2.92
<PER-SHARE-DIVIDEND> 0
<PER-SHARE-DISTRIBUTIONS> 1.45
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<PER-SHARE-NAV-END> 17.66
<EXPENSE-RATIO> 1.93
[AVG-DEBT-OUTSTANDING] 163352
[AVG-DEBT-PER-SHARE] 0.003
</TABLE>
Mid-Cap Fund
3/31/99 NSAR
Responses to Item 77
Sub-Item 77D: Policies with respect to security investments (Y)
(a) None.
(b) None.
(c) None.
(d) None.
(e) Effective February 1, 1999, the registrant may invest in securities of other
investment companies to the extent permitted by Section 12(d)(1)(A) under the
Investment Company Act of 1940.
(f) None.
(g) None.
