I have a form with two subforms. I want to display a total of the hours from the subform's one table. I thought that this should do the trick:

=Sum([Forms]![activityregistration Subform]![hours])

But it gives an error only. Suggestions?

Put the Sum in the subform footer and refer to it on the main form.
Sum([hours])

Sum([Forms]![activityregistration Subform]![newTxtBox])

Thanks Rich. It works with:
=[activityregistration Subform]![newTxtBox])