WARNING: THE EDGAR SYSTEM ENCOUNTERED ERROR(S) WHILE PROCESSING THIS SCHEDULE.

<TABLE> <S> <C>

<PAGE>
 
<ARTICLE> 5
       
<S>                             <C>                     <C>                     <C>
<PERIOD-TYPE>                   03-MOS                   06-MOS                   09-MOS
<FISCAL-YEAR-END>                          DEC-31-1998             DEC-31-1998             DEC-31-1998
<PERIOD-START>                             JAN-01-1998             JAN-01-1998             JAN-01-1998
<PERIOD-END>                               MAR-31-1998             JUN-30-1998             SEP-30-1998
<CASH>                                          10,647                  12,537                  18,509
<SECURITIES>                                   104,721                 105,470                 109,954
<RECEIVABLES>                                   72,376                  78,230                  82,259
<ALLOWANCES>                                   (1,298)                 (1,356)                 (1,386)
<INVENTORY>                                          0                       0                       0
<CURRENT-ASSETS>                               200,294                 205,482                 221,750
<PP&E>                                          57,998                  60,393                  64,079
<DEPRECIATION>                                  28,026                  30,682                  33,521
<TOTAL-ASSETS>                                 242,586                 250,686                 268,372
<CURRENT-LIABILITIES>                           46,861                  47,311                  52,150
<BONDS>                                              0                       0                       0
<PREFERRED-MANDATORY>                                0                       0                       0
<PREFERRED>                                          0                       0                       0
<COMMON>                                           263                     263                     265
<OTHER-SE>                                     186,803                 194,672                 207,238
<TOTAL-LIABILITY-AND-EQUITY>                   242,586                 250,686                 268,372
<SALES>                                         72,737                 152,101                 235,459
<TOTAL-REVENUES>                                72,737                 152,101                 235,459
<CGS>                                           37,248                  77,656                 120,060
<TOTAL-COSTS>                                   27,269                  52,978                  82,436
<OTHER-EXPENSES>                                     0                       0                       0
<LOSS-PROVISION>                                    31                      88                     118
<INTEREST-EXPENSE>                                  43                      43                      48
<INCOME-PRETAX>                                  9,342                  22,848                  36,963
<INCOME-TAX>                                     4,940                  10,340                  15,990
<INCOME-CONTINUING>                              4,402                  12,508                  20,973
<DISCONTINUED>                                       0                       0                       0
<EXTRAORDINARY>                                      0                       0                       0
<CHANGES>                                            0                       0                       0
<NET-INCOME>                                     4,402                  12,508                  20,973
<EPS-PRIMARY>                                     0.17                    0.48                    0.80
<EPS-DILUTED>                                     0.16                    0.46                    0.77
        

</TABLE>