<PAGE>
SEI INSTITUTIONAL MANAGED TRUST
TAX-MANAGED LARGE CAP FUND
SUPPLEMENT DATED APRIL 15, 1999, TO THE
CLASS A PROSPECTUS
DATED JANUARY 31, 1999
THIS SUPPLEMENT PROVIDES NEW AND ADDITIONAL INFORMATION BEYOND THAT CONTAINED IN
THE PROSPECTUS. IT SHOULD BE RETAINED AND READ IN CONJUNCTION WITH SUCH
PROSPECTUS.
---------------------------------------------------------
The following disclosure is inserted on page 24 of the prospectus under the
section entitled Purchasing Fund Shares:
Investors may be charged a fee for purchase and/or redemption transactions
effectuated through certain broker-dealers or other financial intermediaries.
----------------------------------------------------------------
The prospectus is hereby amended to reflect this change.
PLEASE RETAIN THIS SUPPLEMENT FOR FUTURE REFERENCE.
SEI-B-101-05
