yes, but does depend on what is updateable.
If your main form textbox is bound to a recordsource field, in the combobox afterupdate event put the following code:
Parent.controlname=combo
controlname is the name of the text box on your main form
combo is the name of your combo box on the sub form.
Alternatively if your text box is unbound you can use the method above or set the text box controlsource to be:
=[subform].[form].[combo]
Subform is the name of the subform control on the main form, not the name of the subform
