<PAGE>
ALLMERICA INVESTMENT TRUST
(SUPPLEMENT TO PROSPECTUS DATED MAY 1, 1999)
Effective immediately, the sections relating to the Investment Grade Income
Fund, Government Bond Fund, Equity Index Fund and Money Market Fund in the table
containing information about the individuals who are primarily responsible for
the day-to-day management of the Funds' portfolios under "Management of the
Funds" in the Trust's Prospectus are amended to read in their entirety as
follows:
<TABLE>
<CAPTION>
Fund Name and Name and Title of Service with Business Experience
Sub-Adviser Name Portfolio Manager(s) Sub-Adviser for Past Five Years
- ---------------- -------------------- ----------- -------------------
<S> <C> <C> <C>
Investment Grade Income Fund Richard J. Litchfield, 1995-Present He was a mortgage-backed
Allmerica Asset Management, Inc. CFA and Vice President securities analyst and trader
("AAM") at Keystone Investments,
Inc. prior to joining AAM.
Government Bond Fund, Equity John C. Donohue 1995-Present He was a portfolio manager
Index Fund and Money Market Vice President at CS First Boston
Fund Investment Management
Allmerica Asset Management, Inc. prior to joining AAM.
("AAM")
</TABLE>
Dated: March 17, 2000
