>>I would store the primary key in a zero-width column of the listbox.
>Sounds good.
>How can I reference that that column as I am looping thru?

Something like:

FOR i = 1 TO ThisForm.lstBox.ListCount
    IF ThisForm.lstBox.Selected(i)
        liPrimaryKey = VAL(ThisForm.lstBox.List(i,2))
    ENDif
ENDfor