The ultrastructure of the oncospheres of Taenia saginata after treatment with artificial gastric juice is described. After the treatment with the gastric juice, the oncosphere remains in the embryophore. Finger-like processes are formed on the membrane separating the granular layer of embryophore from the oncospheral membrane and micropores arise in the outer unit membrane of the oncospheral membrane. After the treatment with the intestinal juice, the embryophore disintegrates. The oncospheral membrane is again without pores and it is shed off after induction of active movement of the oncosphere. The activity of penetration glands increases: in the first phase, the secretion accumulates beneath the oncospheral membrane and after its shedding there appear granules of other type in the glands. This second phase of secretion seems to be important for the penetration of the oncosphere through the mucosa of the small intestine into the processes. The functional importance of the structural changes observed, the two types of secretion of penetration glands and their relation to pathological changes in the infected organ, and immune reactions are discussed.