Abstract: A TEM study of murine malaria parasites, Plasmodium berghei and P. yoelii was performed by consecutive sampling in vivo to look into the early sequential changes in the ultra-structure of the merozoites after entering red cells. The results showed that once finishing invasion, the merozoite resided in the peripheral cytoplasm of the red cell, creating a bulge at the invasion site, with an additional unit membrane around it (parasitophorous vacuole); apical structures disappeared; the spherical body was degenerative or atrophic and separated from the mitochondrion and nucleus. The mitochondrion became more extended and the nucleus elongated and curved. There were more Er vesicles in the cytoplasm, taking a dilated polyangular shape. The inner double membrane was separated from the outer membrane and got into incomplete, winding, finally disappeared. Sometimes multimembranous bodies could be seen in the peripheral spaces. Once the dedifferentiation process was over, the merozoite was transformed into an early trophozoite , with a single plasma membrane and decreased density. Individual large Er vesicle with acute angles was found in the cytoplasm, and small food pills appeared beneath the plasma membrane; then the shape of the parasite changed from a ball-like one to a pie-like one, gradually the flat cell body rolled up, with its edges met and fused, resulting in the formation of a large food vacuole,with digestive vac-uoles and pigment granules around it. Thus, it grew into a middle-aged trophozoite.