The population structure, growth, migration and participation in reproduction of the Japanese mitten crab Eriocheir japonicus was studied, based on the size and density distribution and variation in sex ratio along the Kaminokawa River, Kagoshima Prefecture, Japan. Crabs were collected using crab baskets between September, 1985 and August, 1986. In the freshwater area, the proportion of large crabs (55 mm≦ CW) increased while the number of crabs per basket (CPUE values) decreased with distance upstream, and the sex ratio also became female-biased with distance upstream. Over one year, the sex ratio among the large crabs was female-biased; among the small crabs it was male-biased. In the brackish-water area, the sex ratio was male-biased, but during the breeding season, a female-bias among large crabs and a male-bias among small crabs was noted. Consequently, we suggest that female crabs migrate upstream in larger numbers than male crabs. During the breeding season, the crabs which participate in reproduction consist of female-biased large crabs migrating downstream from the upper regions as well as male-biased small crabs migrating from the lower regions of the river.