Gonadal development, reproductive cycle, gonad index, meat weight rate, and first sexual maturity of the top shell, Omphalius rusticus were Investigated monthly by histological observations. Specimens were collected from the west coast of Korea during the period from January to December in 1999. O. rusticus is dioecious and oviparous. The gonad is widely situated on the surface of the digestive g1and located in the posterior spiral meat part in the shell. The ovary and the testis were composed of a number of oogenic follicles and several spermatogenic follicles, respectively. Ripe oocytes were approximately 120-130 $mu$m in diameter. The meat weight rate peaked in June (27.7%), and then rapidly decreased in September (19.5%). Monthly changes in the gonad index in both sexes reached the maximum in June, and then sharply decreased in September. Percentages of first sexual maturity of female and male snails ranging from 9.0 to 9.9 mm in shell heights were 58.3% and 54.5%, respectively, and 100% in those over 11.0 mm in both sexes participated in reproduction. Reproductive cycle of this species can be categorized into five successive stages: in females, early active (October to April), late active (December to June), ripe (April to September), spawning (July to September) and recovery (September to January): in males, early active (November to March), late active (December to June), ripe (April to September), spawning (July to September) and recovery (September to December). Gonadal development, gametogenesis, reproductive cycle, and spawning were closely related to the seawater temperature.