ABSTRACT Gonadosomatic index (GSI), annual reproductive cycle, monthly changes in oocyte sizefrequency distributions, size at first sexual maturity and sex ratio of the sun and moon scallop, Amusium japonicum japonicum, were investigated. Samples were collected monthly from the subtidal zone of Sogwipo, Jejudo, Korea. Amusium japonicum japonicum is dioecious. The ovary is composed of a number of oogenic follicles, and the testis consists of a number of acini. Monthly variation in the GSI showed a pattern similar to that of the annual reproductive cycle. Ripe oocytes were about 70–90 µ in diameter. The spawning period was from November to January, and the main spawning occurred between December and January when the seawater temperature was relatively low. The annual reproductive cycle of this species can be classified into five successive stages: early active stage (April to June), late active stage (June to September), ripe stage (October to November), spawning stage (November to January), and spent / res...