The ultrastructures of germ cells and follicle cells during oogenesis, the reproductive cycle, and the size at sexual maturity in female Meretrix lusoria were investigated for clams collected from Simpo, Korea by cytological, histological, and morphometric analysis. Vitellogenesis occurred through a process of autosynthesis, involving the combined activity of the Golgi complex, mitochondria and rough endoplasmic reticulum, and a process of heterosynthesis, in which extraovarian precursors were incorporated into oocytes by endocytotic activity at the basal region of the oolemma of the early vitellogenic oocytes. The follicle cells appeared to play an integral role in vitellogenesis and oocyte degeneration: the functions of the follicle cells are phagocytosis and intracellular digestion of products originating from the degeneration of oocytes. During the period of oocyte degeneration, follicle cells of this species probably have a lysosomal system for breakdown, and resorb various phagosomes in the cytoplasm for nutrient storage, as seen in other bivalves. The reproductive cycle in females can be classified into five successive stages: early active stage (January to March), late active stage (February to May), ripe stage (April to August), partially spawned stage (June to September), and spent/inactive stage (September to February). The spawning period continues from June to September, with a peak between July and August when the seawater temperature exceeds 22°C. The percentage of individual female clams 40.0–45.0 mm in shell length that were at first sexual maturity was over 56.3%, and for clams over 50.0 mm in shell length, it was 100%. Shell length at 50% of sexual maturity (rate of sexual maturity, RM₅₀) was 41.0 mm. Thus, female clams were determined to be sexually mature at two years of age. Because harvesting clams <40.1 mm in shell length could potentially cause a drastic reduction in recruitment, a measure indicating a prohibitory fishing size should be taken for adequate fisheries management.