The optical transmission of CdS single crystals has been changed using a laser. With ac techniques, these changes have been observed from 530 to 2000 nm in the transmission spectrum, and include both increased and decreased transmission. Comparison between the intensity dependence and the temperature dependence of photoconductivity and of change of transmission suggest a mechanism. The effect is a change of optical absorption produced primarily by the change in hole population of imperfection levels in the band gap. Changes in transmission due to population changes of approximately 1012cm−3have been observed.