Field studies on the reproductive traits of a population of a gnathiid isopod Elaphognathia cornigera (Nunomura) were conducted at a rocky intertidal shore on the Izu Peninsula, southern Japan. Fortnightly surveys for two years showed some peculiar seasonal trends in the female reproductive characteristics. While adult males and juveniles of the species occurred throughout the year, semelparous adult females occurred only from spring to autumn. Based on the occurrence patterns of the females in six different reproductive stages, there seemed to be four generations in 1994 and three in 1995. Though the reproductive investment of a female showed no significant variation through each year, there were marked seasonal variations in the female broods. Female body size and brood size became the largest in early summer, but egg size reached its maximum in autumn. Maximum food availability (high density of gobies) in spring and the unfavorable environmental conditions in winter were thought to be the major controlling factors of the size and number of female broods.