The owl monkey (Aotus trivirgatus) is the only nocturnal monkey. The photopigments of Aotus and the relationship between these photopigments and visual discrimination were examined through (1) an analysis of the flicker photometric electroretinogram (ERG), (2) psychophysical tests of visual sensitivity and color vision, and (3) a search for the presence of the photopigment gene necessary for the production of a short-wavelength sensitive (SWS) photopigment. Both electrophysiological and behavioral measurements indicate that in addition to a rod photopigment the retina of this primate contains only one other photopigment type--a cone pigment having a spectral peak ca 543 nm. Earlier results that suggested these monkeys can make crude color discriminations are interpreted as probably resulting from the joint exploitation of signals from rods and cones. Although Aotus has no functional SWS photopigment, hybridization analysis shows that Aotus has a pigment gene that is highly homologous to the human SWS photopigment gene.