Abstract: Adult Schistosoma japonicum was cultured in 851 medium for 23 days before ultrastructural observation. The ultrastructures of the reproductive organs of the adult worms reveal that some oocytes in the ovary degenerate or become necrotic. The mature vitelline cells also degenerate; some vitelline droplets have fused. On the other hand, the number of the lipid droplets and their volume increase. The eggs formed during the posterior period of culture are devoid of eggshells and ova. However, the reproductive organs, such as the testes in male and the ootype, Mehlis'gland and uterus in female seem to maintain their normal structures. It indicates that the female reproductive organs, especially the ovary and vitelline gland are most sensitive to the conditions of culture in vitro. The results may explain why the function of egg-laying declines and the percentage of the abnormal eggs increases in the posterior culture period, and also may provide clues to improve our culture system. The ultrastructure of the eggs demonstrate that there are belt-shaped zones of high-electron density alternating with zones of low-electron density in the eggshell of the eggs from early reproductive stage. The possile function of the structure is briefly discussed.