Abstract: The cultured vitelline cells from adult Schistosoma japonica were observed ultrastructurally.The cells were more sensitive than somatic cells to the changes of in vitro cultured condition, and degenerated in different degrees along with the prolongation of culture.The degeneration was observed earier in mature vitelline cells than in imrnature ones.Changes in the cell structure caused by the degeneration were examined with the following characteristics: the electron density of cells became lower,vitelline globules were fused with each other, the space between vitelline globules and the membrane became larger gradually, vitelline globules were released and uncovered at last.The number and volume of lipid droplets increased.Rough-surfaced endoplasmic reticula enlarged and became vacuolar, and ribosomes left the surface of them.Therefore it was considered that ultrastructural changes of cultured immature vitelline cells could be used to estimate whether the cultured conditions were appropriate.