This study examines the spectral sensitivity and cone topography of the sockeye salmon Oncorhynchus nerka throughout its life history with special emphasis on ultraviolet sensitivity. Electrophysiological recordings from the optic nerve show that ultraviolet sensitivity is greatly diminished at the smolt stage but reappears in adult fish weighing about 201 g. Concomitantly, light microscopy observations of the retina show that ultraviolet cones disappear from the dorsal and temporal retina at the smolt stage but reappear at the adult stage. These changes occur for sockeye salmon raised in fresh water or salt water after smoltification. In contrast to this ultraviolet cycle, the other cone mechanisms (short-, middle- and long-wavelength-sensitive) and the rod mechanism remain present throughout ontogeny. The natural appearance and disappearance of ultraviolet cones in salmonid retinas follows surges in blood thyroxine at critical developmental periods. Their presence coincides with times of prominent feeding on zooplankton and/or small fish that may be more visible under ultraviolet light. It is proposed that the primary function of ultraviolet cones in salmonids is to improve prey contrast.