The behavior of juveniles of the Japanese endemic crayfish Cambaroides japonicus (Decapoda: Cambaridae) was observed in the field and in the laboratory. In addition, the occurrence and placement of spermatophores on adult females is described from specimens inhabiting small brooks at Otaru and Hamamasu, Hokkaido, Japan, sampled from 1998 to 1999. The second stage juveniles left their mother permanently after some exploration behavior. In adult females, spermatophores were attached to the surface of the seminal receptacle (annulus ventralis), which is situated at the posterior margin of the seventh thoracic sternite. In C. japonicus the invagination of the annulus ventralis is not deep enough to hide the male spermatophores. Previous studies of the Astacidae reported that juveniles left their mother at the second stage, that spermatophores were attached to the surfaces of sternal plates of the last three pereopods, and that an annulus ventralis did not exist. In contrast, Cambaridae juveniles normally detach from the mother at the third stage, but they may detach and stay longer with the parent until the fourth stage. There is variability in this trait. The spermatophores are completely hidden in the annulus ventralis of adult females. Therefore, although C. japonicus belongs to the family Cambaridae, the juvenile stage that detaches from the mother is the same as that reported in Astacidae. The position of the spermatophore attached to the adult female exhibits an intermediate state between that of the Astacidae and that of the Cambaridae.