ZnSe has been deposited on (100) GaAs by molecular beam epitaxy under a UV irradiance of 4 W cm-2. At this level, the ZnSe growth rate can be severely reduced or stopped altogether. We show for the first time that the reduction in growth rate is a strong function of layer thickness for epilayers less than approximately 2000 A thick. The growth rate under laser irradiation is found to be directly proportional to the Se flux, demonstrating that Se is the minority surface species even under high VI-II flux ratios. Furthermore we present observations indicating that the electron stimulated desorption of Se by RHEED beams and the photostimulated desorption are both due to the accumulation of photoinduced holes at the growth surface.