Observations on the negative resistance of insulating crystals of CdS with indium cathodes and gold anodes are reported. With applied voltages below the critical voltage for the onset of negative resistance a crystal can be switched from a low current to a high current state by light of wavelengths shorter than 8100 Å. This effect is attributed to the presence of hole traps about 1 eV above the valence band. With pulsed applied voltages the current shows a steep rise to a highly conducting state after an induction time during which holes are being accumulated at the hole traps.At low temperatures visible light is emitted when a crystal is in the highly conducting state. The spectral distribution of this radiation is compared with the photoluminescence.