The sweet potato weevil, Cylas formicarius (Fabricius), is one of the most important pests of the sweet potato, Ipomoea batatas (L.), throughout the tropical and subtropical regions of the world. We investigated the winter-time mating and oviposition of C. formicarius on the Amami Islands, Japan, the northernmost area of C. formicarius's distribution. The percentage of mated females was as low as about 30% from late December to late January. Then it increased after February, and reached more than 80% in April. Since the wild males were able to mate even in winter, suppression of mating during winter might be caused by the fact that none of the females released the sex pheromone during winter. No wild females were found to develop their ovarian follicles or have mature eggs from late November to late January but began these developments only after late February. However, the proportions of females with mature eggs in late February greatly differed year by year. The number of eggs laid by wild weevils on the sweet potato tuberous root traps was gradually decreased after October and remained very low from late December to mid February. These results showed that the wild females hardly laid any eggs in winter.