We investigated the winter-time mating and oviposition of Cylas formicarius (Fabricius) on Amami-Oshima Island and Kikai Island, Japan. No wild females were found to develop their ovarian follicles or have mature eggs from late November to late January. The ratio of mated females was as low as about 30% from late December to late January. Females began to develop their ovarian follicle after late February, thereby leading to an increased mating ratio. As for the wild males, they seemed to be able to mate even during winter. However, since none of the females seemed to release sex pheromones at this time, winter-time mating hardly occurred in the field. The number of eggs laid by the wild weevil was very low from December to February and then gradually increased in late February. Those females that had emerged during the winter began to lay eggs in late March.