The spawning season of the Japanese mantis shrimp Oratosquilla oratoria (De Haan) was examined in Ishikari Bay. The spawning season was estimated based on the temporal changes of female gonad somatic index (GSI) and sex ratio (number of female individuals/total number of individuals×100(%)) obtained from data between 1989 and 1991. The female GSI gradually increased from April to June, decreased in July, and began to increase again after October until November. However, GSI in November did not attain the value in the next spring. The sex ratio was low between mid June and July and high in May and November. Changes of sex ratio show the difference between male and female activities outside their burrows. The extreme low sex ratio is caused by the egg nursing behavior of females in the burrow. The spawning season of O. oratoria in Ishikari Bay was between June and July and once a year.
