>When I say "temp file" I mean that the file is a .dbf that was created as the result of an SQL-SELECT. Each time the program is run, the SQL-SELECT that generated it is executed.

Hmm...no filter or filtered index, no buffering, no locking...is there a FOR clause in the REPLACE that might be causing trouble?

And are you sure the table is selected? Try this:

REPLACE ALL field WITH somevalue IN mytable