Ok, ok, i've understand that I've to modify the "Befor Update" of the form.
I've put a code like this, could it be right ??
Private Sub Numero_Protocollo_BeforeUpdate(Cancel As Integer)
Me.txtNumero_Protocollo = Nz(DMax("[Numero_Protocollo]", "[Protocolli_in]"), 0) + 1
End Sub
It seems it...