Intense colored light from a gas laser evokes a photoelectric potential in the photoreceptor cells of the ventral eye of Limulus. This potential has two components, both of which have the action spectrum of a rhodopsin with an absorbancy maximum at 530 nm. The evidence is consistent with the hypothesis that the photoelectric potential arises directly from the orderly array of rhodopsin molecules which are an integral part of the photoreceptor cell membrane.