Abstract: The ultrastructural changes during invasion into erythrocyte by Plasmodium berghei and P. yoelii were investigated with TEM in vivo. The results showed that both mero-zoites and red cells could actively accommodate themselves to approach each other. After contact, the erythrocyte formed an invagination to surround the parasite. In the meantime, the merozoite kept firm association with the wall of the invagination in two ways: a fixed adhesion with its apical end to the base, where the confronted portion of the red cell membrane is disorganized, and a moving junction on the circumferential surface. The coat was generally lost within the invagination, but residual bristles could be seen sometimes. Rhoptries became more rounded, expanded, and less dense in the early phase, then vesiculated, finally disappeared. Aggregated intranuclear ribosomes and vacuoles were seen in the nucleus. In the erythrocyte, a few oval vacuoles occurred near the invagination.P. y. yoelii merozoites preferred relatively young erythrocytes and were made to form a constriction by the tightened orifice of the invagination. On the contrary, P. berghei parasites were all observed in mature ones without constrictions.