The daily periodicity of sex pheromone emission by females of the rice leaf bug, Trigonotylus caelestialium, was examined. Male bugs were captured at any time of the day in traps baited with 10 virgin females placed in a field overgrown with weeds. In the laboratory, thirty pairs of virgin adults three days after emergence were released in the rearing cage at different times of day, and the number of mated females was checked 4 h later. The percentage of mated females was more than 60% regardless of the time of release and the light conditions. These results suggest that there is no daily periodicity in sex pheromone emission by females and mating.