To estimate the biological function of the male-produced pheromone of Piezodorus hybneri, we investigated the relationship between sexual maturity and pheromone production. Copulation was first observed at day 4 after emergence of males. The males showed high mating activity between days 5 and 15. Development of the ectodermal accessory gland (EAG) was also investigated as an indicator of male sexual maturity. The EAG, which was entirely immature at adult emergence, developed gradually between days 3 and 10. Males started to produce pheromones at days 3 to 5 after adult emergence, and production became maximal at days 5 to 10, remaining at a high level until day 30. No pheromone components were detected in any diapause males, which showed neither mating behavior nor EAG development. These results indicate that P. hybneri males produce their pheromone simultaneously with the development of sexual maturity, and thus the function of this pheromone seems to be related to sexual behavior.