<PAGE>
ALLMERICA INVESTMENT TRUST (THE "TRUST")
(SUPPLEMENT EFFECTIVE AUGUST 21, 2000 TO PROSPECTUS DATED MAY 1, 2000)
Effective October 1, 2000, the information relating to the Core Equity Fund and
the Money Market Fund in the tables under the heading, "Fees and Expenses of the
Funds", in the section entitled, "Expense Summary", in the Prospectus is amended
as follows:
<TABLE>
<CAPTION>
Shareholder Annual Fund Operating Expenses Total Annual
Fees (expenses deducted from Fund assets) Fund
(fees paid directly Management Distribution Other Operating
from your investment) Fees (12b-1) Fees Expenses Expenses
--------------------- ----------- ------------- -------- --------------
<S> <C> <C> <C> <C> <C>
Core Equity Fund None 0.53%** None 0.05% 0.58%(1),(2)**
Money Market Fund None 0.31%** None 0.05% 0.36%(1)**
</TABLE>
** Effective October 1, 2000, the management fee rates for the Core Equity
Fund and Money Market Fund have been revised. The expense information in the
table has been restated to reflect current fees.
The percentage shown in footnote (2) under the heading, "Fees and Expenses of
the Funds", in the section entitled, "Expense Summary", in the Prospectus is
changed to 0.55% for the Core Equity Fund.
The information relating to the Core Equity Fund and the Money Market Fund in
the table under the heading, "Example", in the section entitled, "Expense
Summary", in the Prospectus is amended as follows:
<TABLE>
<CAPTION>
1 Year 3 Years 5 Years 10 Years
------ ------- ------- --------
<S> <C> <C> <C> <C>
Core Equity Fund $59 $186 $325 $727
Money Market Fund $37 $116 $202 $456
</TABLE>
