Optomotor reaction has been studied much to examine the visual function of fish. In this study, the optomotor reaction was utilized in order to measure the spectral sensitivity of fish under scotopic conditions, in which two species of cobaltcap silverside and medaka were investigated. Light of various wavelength was irradiated for the fish which had been fully adapted in the dark and the optomotor reaction of the fish was observed. The optomotor reaction developed more slowly in the darkadapted fish than in fish adapted in the irradiating light. When light of a wavelength giving an absorption maximum for the rod visual pigments of the fish was irradiated, the inhibition of the optomotor reaction was greatest. Thus, the spectral sensitivity curve for the optomotor reaction of dark-adapted fish coincided well with the absorption spectrum of the rod visual pigments in both the species of fish. The usefulness of this behavioral method for ascertaining the spectral sensitivity of fish under scotopic conditions was discussed.
