Using intersect in subquery

I have tried to use intersect in subquery causing the following error.
Please help!
ERROR:  parser: parse error at or near "intersect"


select * from Table1
where KeyField1 in
(select KeyField2 from Table2 where  somecondition
intersect
select KeyField3 from Table3  where somecondition);

(I believe) I am running PG 6.4.  The INTERSECT by itself works, but not in
a subquery.

Thanks!

hamid