I agree with Daniel, it seems that your index is corrupted. Just for the sake of an example, try adding NOOPTIMIZE to your LOCATE calls (this will prevent FoxPro from using Rushmore) and see what happens.

>Now this is weird :(
>After further experimentation I found the following:
>If I open table A and ask for a LOCATE on the primary index integer field it works; If I open table B and ask for a LOCATE on the primary index integer field, it DOESN'T work; if in table B I write
>LOCATE FOR ALLTRIM(STR(INTEGERFIELD))= "NUMERICVALUE" it works!!!. I am more confused than ever.
>Anyone has an idea what's going on?
>Thanks in advance.