Ultrastructures and their dynamic changes of the cultured cells from Schistosoma japonicum were observed in the present experiments. Several types-including polygonal, round granular, deltaic fan-shaped and flagellated cells—were found in the cultures. The polygonal cells took a major ratio in the cultures from adult S. japonicum, while the majority from schistosomula was round granular cells. The ultrastuctures on the cell surface were different between the cells from adults and schistosomula. Some papilla-like tubercula, microvilli and pinocytotic vesicles were observed on the surface of adult cells, but none were found on schistosomula cells. However, more or less mitochondria, endoplasmic reticula, ribosomes and glycogen were observed in the cytoplasm of the cultured cells from both adults and schistosomula. Golgi complexes were rarely found. The nucleus was round, with round nucleolus inside and clear pores on the unit membrane. There was much lumpish heterochromatin located near to the nuclear membrane. Cells from different worm tissues had their own organelles. The germ cells, vitelline cells, flame cells, multinucleate subtegumental cells and nerve cells could be observed in the cultures from adults. The vitelline cells were the greatest in number and nerve cells were the least in number among them. Similarly, there were germ cells, sustentacular cells, flame cells, nerve cells, mast cells, muscle cells, multinucleate subtegumental cells, interstitial cells and penetration gland cells in the cultures from the schistomomula. In addition, a few division cells were also found. It indicated that the schistosomula cells had greater potential ability to proliferate than the adult cells in in vitro culture. Along with the prolongation of the culture time, degeneration of schistosomal cell occurred more and more. Generally, the electron density of cultures gradually got lower, the cristae of mitochondria blurred and disappeared and the mitochondria themselves swelled and finally vacuoled completely. Vitelline cells were most sensitive to the changes of the in vitro condition in all cultures. Their degeneration showed the following characteristics: (1) vitelline globules fused each other, the space between vitelline globules and the membrane surrounding them broadened gradually and vitelline globules were released and uncovered; (2) rough-surfaced endoplasmic reticula enlarged, vacuolated and the ribosomes dropped; and (3) the number and volume of lipid increased. The ultrastructural changes of most of the cultures from schistosomula had the following trends: (1) heterochromatin increased and euchromatin decreased gradually; and (2) endoplasmic reticula changed into short tubes and vacuoles and disappeared finally. The degenerative process of the cultures from S. japonicum consisted of necrosis according to the ultrastructural changes of the mitochondria, vitelline globules, chromatin and endoplasmic reticula within the cells. The changes of the above structures could be used to estimate whether the culture conditions were appropriate.