The mating behavior and season, with no form alternation, of the Japanese crayfish, Cambaroides japonicus (De Haan, 1841) (Cambaridae), was observed in Lake Komadome, Hokkaido, Japan, from 1993 to 1995. Mating pairs and females with spermatophores were found in September and October in all three years, when water temperature decreased from 20.0° to 12.5°C. Females spawn the following year, and there are six months between mating and spawning. The lengths of each mating pair were similar for both male and female. Mean carapace length in males was 21.7 mm (range: 18.6–24.9), and in females it was 20.9 mm (18.4–22.9). The male lies ventral side up below the female. The male does not use his chelae to hold the female, but rolls his tail over the abdomen of the female. Cambaroides japonicus is placed in the family Cambaridae. All members of Cambaridae in North America have form alternation (cyclic dimorphism), but C. japonicus exhibited no form alternation in its natural habitat.