A longitudinal design was applied in a study of the development of individual differences in aggression, and their relationship to growth and sexual differentiation in the Midas cichlid, Cichlasoma citrinellum. Weight and standard length measurements, as well as several measures of aggression, were obtained at regular intervals. Size ranks within groups were stable over a period spanning an early juvenile phase through sexual maturity. Two of three aggression rank scores were also stable over this interval, but they did not relate in any direct way to social rank. As juveniles, males showed higher levels of aggression than females, but the reverse was true in adults. This sex-specific developmental change resulted both from an increase in female aggression at sexual maturity and a decline in male aggression at sexual maturity. Individuals showed distinct and stable behavioral profiles with respect to the combined aggression tests.