Is the listbox on a worksheet?
If yes, is the listbox from the Forms toolbar?
Then rightclick on the listbox and choose: format control
Then on the Control tab, choose Multi in the selection type frame.
If the listbox is from the control toolbox toolbar, then
go into design mode (another icon on that control toolbox toolbar
Rightclick on the listbox, choose properties
Change the .multiselect property to 1 - fmMultiSelectMulti