>SELECT cs.* FROM CustSer cs LEFT JOIN Contlist cl ;
> ON cs.CustSerID = cl.CustSerID ;
> WHERE cs.CustSerID # cl.CustSerID
>
>I realize I could use a sub-select query, but the time it takes to run is unacceptable (20 minutes).

It's hard to imagine why the following would be slow:

SELECT cs.* FROM CustSer cs ;
WHERE cs.CustSerID NOT IN ( ;
SELECT CustSerID FROM Contlist)

Have you tried running this with SYS(3054, 11) to see what's optimized and what's not?

Tamar