Factors affecting egg size variation and its fitness consequences were studied within and among generations of a multivoltine bean weevil, Bruchidius dorsalis (Fahraeus), which exhibits reversed courtship roles. Because the toughness of host plant (Gleditsia japonica Miquel) seed pods varies among weevil generations in the field and it was supposed that seed hardness varies as well, it was hypothesized that female weevils regulate egg size according to seed hardness. To test this hypothesis, egg size was compared among three generations. The overwintering generation of B. dorsalis laid significantly larger (≈ 40% greater volume) eggs than did the other two generations. To assess the effect of paternal nutritional investment on egg size, females were allowed to mate with either well-fed or poorly fed males. During the first half of the 20-d experiment, no significant effect was detected, but during the second half, females that had mated with well-fed males laid significantly and slightly larger (≈4% greater volume) eggs than those mated with poorly fed males. There was much less variation within generations than among generations. Egg size and the drilling ability of hatched larvae were significantly, positively correlated, indicating that larvae that hatched from larger eggs could drill farther into host plant seeds. These results suggest that female weevils regulate egg size according to seasonal changes in the hardness of host plant seeds, and that paternal investment affects within-generation variation in egg size.