Mark,
I got the same reults with your sample.
But if I just add =.T. at the end it works fine in VFP7 as expected,
no matter if its indexed or not.

SELECT tbl1.*;
FROM tbl1;
LEFT JOIN tbl2;
ON;
tbl1.userid = tbl2.userid;
WHERE;
ISNULL(tbl2.userid) = .T.

Peter