Summary.1. The relation of light to the sexual development of the minnow(Phoxinus IsevisLinn.) has been investigated by restricting the daily light periods in the early months of the year. Groups of fish were kept either in total darkness or in seven hours of light per day.2. In the female fish, these experimental conditions, although not entirely. preventing the development of the ovaries, caused a delay in that development.This delay was more marked in the fish kept in total darkness than in those allowed seven hours of light daily. A blind female fish possessed very poorly developed ovaries.3. In the male fish, a slight delay in development was discernible in the experimental groups in April, but all fish had reached full breeding condition by June.4. As other external factors may be eliminated, these results are interpreted as indicating that an internal reproductive rhythm exists within the minnow, and that this internal rhythm is capable of acting in the absence of those external seasonal changes which may normally reinforce it and render it more precise in its time of action.