<ARTICLE> 6
<CIK> 0000863334
<NAME> PIONEER MID-CAP VALUE FUND

   <NUMBER> 004
   <NAME> PIONEER MID-CAP VALUE FUND CLASS Y
<CURRENCY> US

                             
<PERIOD-TYPE>                   6-MOS
<FISCAL-YEAR-END>                          OCT-31-2000
<PERIOD-END>                               APR-30-2000
<EXCHANGE-RATE>                                      1
<INVESTMENTS-AT-COST>                       1250671936
<INVESTMENTS-AT-VALUE>                      1370523337
<RECEIVABLES>                                 25112373
<ASSETS-OTHER>                                    1114
<OTHER-ITEMS-ASSETS>                               421
<TOTAL-ASSETS>                              1395637245
<PAYABLE-FOR-SECURITIES>                      10030935
<SENIOR-LONG-TERM-DEBT>                              0
<OTHER-ITEMS-LIABILITIES>                      4873616
<TOTAL-LIABILITIES>                           14904551
<SENIOR-EQUITY>                                      0
<PAID-IN-CAPITAL-COMMON>                    1236927369
<SHARES-COMMON-STOCK>                           191279
<SHARES-COMMON-PRIOR>                           199658
<ACCUMULATED-NII-CURRENT>                      4248162
<OVERDISTRIBUTION-NII>                               0
<ACCUMULATED-NET-GAINS>                       19705762
<OVERDISTRIBUTION-GAINS>                             0
<ACCUM-APPREC-OR-DEPREC>                     119851401
<NET-ASSETS>                                1380732694
<DIVIDEND-INCOME>                              9092261
<INTEREST-INCOME>                              1055315
<OTHER-INCOME>                                       0
<EXPENSES-NET>                                10004404
<NET-INVESTMENT-INCOME>                         143172
<REALIZED-GAINS-CURRENT>                      22927757
<APPREC-INCREASE-CURRENT>                    112485818
<NET-CHANGE-FROM-OPS>                        135556747
<EQUALIZATION>                                       0
<DISTRIBUTIONS-OF-INCOME>                            0
<DISTRIBUTIONS-OF-GAINS>                        523675
<DISTRIBUTIONS-OTHER>                                0
<NUMBER-OF-SHARES-SOLD>                          21146
<NUMBER-OF-SHARES-REDEEMED>                      58521
<SHARES-REINVESTED>                              28996
<NET-CHANGE-IN-ASSETS>                     (135295321)
<ACCUMULATED-NII-PRIOR>                        4104990
<ACCUMULATED-GAINS-PRIOR>                    198143572
<OVERDISTRIB-NII-PRIOR>                              0
<OVERDIST-NET-GAINS-PRIOR>                           0
<GROSS-ADVISORY-FEES>                          4032666
<INTEREST-EXPENSE>                                   0
<GROSS-EXPENSE>                               10136956
<AVERAGE-NET-ASSETS>                           3883995
<PER-SHARE-NAV-BEGIN>                            19.91
<PER-SHARE-NII>                                   0.08
<PER-SHARE-GAIN-APPREC>                           1.83
<PER-SHARE-DIVIDEND>                                 0
<PER-SHARE-DISTRIBUTIONS>                         2.64
<RETURNS-OF-CAPITAL>                                 0
<PER-SHARE-NAV-END>                              19.18
<EXPENSE-RATIO>                                   0.63