<TABLE> <S> <C>

<ARTICLE> 6
<CIK> 0000078713
<NAME> PIONEER FUND
<SERIES>
   <NUMBER> 004
   <NAME> PIONEER FUND CLASS Y
<CURRENCY> US

<S>                             <C>
<PERIOD-TYPE>                   YEAR
<FISCAL-YEAR-END>                          DEC-31-1999
<PERIOD-END>                               DEC-31-1999
<EXCHANGE-RATE> 1
<INVESTMENTS-AT-COST>                       3803784231
<INVESTMENTS-AT-VALUE>                      7388882779
<RECEIVABLES>                                 23159363
<ASSETS-OTHER>                                   85248
<OTHER-ITEMS-ASSETS>                                13
<TOTAL-ASSETS>                              7412127403
<PAYABLE-FOR-SECURITIES>                             0
<SENIOR-LONG-TERM-DEBT>                              0
<OTHER-ITEMS-LIABILITIES>                     11765319
<TOTAL-LIABILITIES>                           11765319
<SENIOR-EQUITY>                                      0
<PAID-IN-CAPITAL-COMMON>                    3815895582
<SHARES-COMMON-STOCK>                           259313
<SHARES-COMMON-PRIOR>                                0
<ACCUMULATED-NII-CURRENT>                            0
<OVERDISTRIBUTION-NII>                               0
<ACCUMULATED-NET-GAINS>                              0
<OVERDISTRIBUTION-GAINS>                        629635
<ACCUM-APPREC-OR-DEPREC>                    3585096137
<NET-ASSETS>                                7400362084
<DIVIDEND-INCOME>                             94221864
<INTEREST-INCOME>                              2511671
<OTHER-INCOME>                                       0
<EXPENSES-NET>                                75148005
<NET-INVESTMENT-INCOME>                       21585530
<REALIZED-GAINS-CURRENT>                     325811540
<APPREC-INCREASE-CURRENT>                    609475052
<NET-CHANGE-FROM-OPS>                        956872122
<EQUALIZATION>                                       0
<DISTRIBUTIONS-OF-INCOME>                        58721
<DISTRIBUTIONS-OF-GAINS>                        546104
<DISTRIBUTIONS-OTHER>                                0
<NUMBER-OF-SHARES-SOLD>                         265515
<NUMBER-OF-SHARES-REDEEMED>                      19090
<SHARES-REINVESTED>                              12888
<NET-CHANGE-IN-ASSETS>                      1692416982
<ACCUMULATED-NII-PRIOR>                              0
<ACCUMULATED-GAINS-PRIOR>                            0
<OVERDISTRIB-NII-PRIOR>                              0
<OVERDIST-NET-GAINS-PRIOR>                           0
<GROSS-ADVISORY-FEES>                         43260917
<INTEREST-EXPENSE>                                4312
<GROSS-EXPENSE>                               75997850
<AVERAGE-NET-ASSETS>                          11258905
<PER-SHARE-NAV-BEGIN>                            46.44
<PER-SHARE-NII>                                   0.23
<PER-SHARE-GAIN-APPREC>                           3.41
<PER-SHARE-DIVIDEND>                              0.24
<PER-SHARE-DISTRIBUTIONS>                         2.22
<RETURNS-OF-CAPITAL>                                 0
<PER-SHARE-NAV-END>                              47.62
<EXPENSE-RATIO>                                   0.72


</TABLE>