Thanks for pointing me in the right direction!

The problem wasn't that I had bound the combo to a memvar. It was the value I was storing to that memvar, in my attempt to change the combo's value. I was only storing a value for the first part of the combo's row source.

Now I've changed the first combo's Valid code to a) set Combo2.Value rather than storing a value to its memvar (control source), and b) store a value matching the entire row-source expression rather than just the first half. The second combo now shows the selection as I expeted!

Just for grins, I put the memvar (which I'd also removed on your advice) back into the second combo's control source property, and it made no difference. But I'm taking it back out. As you pointed out, there's no reason for binding the combos to memvars.

Thanks,
Rich Addison
ERW, Inc., Southfield, MI