finding rows in one table not in another

I need to find the set of rows in one table that are not in another
table.  I used a query like the following:

    select distinct id from mytable1 where id not in (select distinct id from mytable2);

However, this revealed no rows, even though I know of instances where
id from mytable1 is absent from id in mytable2.  Is there some other
way to do this?  Am I missing something?  Is there a workaround?

Cheers,
Brook