AbstractSeasonal variation in the circadian periodicity of mating behaviour in Chilo suppressalis (Wlk.) was investigated. In the field, mating was initiated at dusk in June, but not until after dark in August. The times of females calling and of male catches in a pheromone trap were later in August than in June. The seasonal variation in time of male catches coincided approximately with that of mating, suggesting that latter may be attributable in part to seasonal changes in the response of males to the sex pheromone. Also, it is likely that these variations are affected by environmental factors, especially temperature.