The gonad index, gonadosomatic index, the condition index, reproductive cycle, first sexual maturity, and sex ratio of the hard shelled mussel, Mytilus coruscus were investigated by using histological analysis and morphometric data. Specimens were collected monthly in Kyeokpo, western Korea from January to December, 2006. The gonad index reached a maximum in February, and minimum in September. Both the monthly variations of the gonad index (GI) and the gonadosomatic index (GSI) reached a maximum in February. After that, their values continually decreased between March and April because of spawning. The monthly variation of the condition index coincides with the GI and the GSI. The spawning period was from February to April, with the main spawning occurring between February and March. The reproductive cycle of this species could be classified into six stages: early active stage (November to January), late active stage (November to February), ripe stage (December to April), partially spawned stage (February to April), degenerative stage (April to October), and resting stage (June to December ). Percentages of sexual maturity was over 50% for the female and male hard shelled mussels that ranges from 40.1 to 50.0 mm in shell length. The percentage was 100% for those that are over 50.1 mm. The sex ratios of females to males over 40.1 mm in shell length were not significantly different from a 1:1 sex ratio (χ2 = 0.63, p > 0.05). No evidence of hermaphroditism was found in histological sections of any hard shelled mussel examined.
