Sylvia-

>I want a validation rule for my Activity.employeeno field that checks against Employee.employeeno for such an employeeno.

Is this a one-time validation? If not, you might want to consider data entry from a CBO of a list of valid employees so that "bad" data never get's input. Also, you can add RI between the tables to ensure validity.

If you want to use a field-level validation, you can create a stored procedure that does the search and then call that procedure from the field validation rule. Note, you probably want to first manually validate this in the table.

You don't ask about this, but I'd caution you to not use the employee number as a key field. There have been many discussions here about using surrogate vs. natural keys.