Entry of merozoites of Plasmodium berghei yoeli and of P. gallinaceum into erythrocytes has been elucidated by electron microscopy. Merozoites approach host cells with the conoid leading the way. At the point of contact between the anterior pole of the parasite and the host cell a focal depression of the red cell membrane forms, which deepens as the merozoite advances. The continuity of the host cell membrane is not disrupted. With deeper invagination of the red cell membrane the resulting cavity conforms to the shape of the merozoite. The site of initial parasite contact forms a relatively constricted orifice through which the posterior portion of the parasite passes. Later the edges of this orifice fuse and the parasite now lies in a vacuole inside the host cell. At this stage the merozoite undergoes a transformation and dedifferentiation. The pellicle loses the characteristic thick inner membrane. The parasite becomes rounded and the conoid and paired organelles are no longer discernible. Trophozoite development is initiated. This sequence of events unequivocally establishes the origin of the parasite membrane envelope and the origin of the space separating the two membranes. The outer membrane is derived from the plasmalemma of the host red cell; the inner is the covering of the parasite.