Abstract: The ookinete formation and mature ookinete of Plasmodium yoelii yoelii were described. During the development from zygote to ookinete, at first a finger-shaped or a stick-shaped projection protruded from one side of zygote (Fig. l).The anterior end of the projection assumed a truncated cone shape (Figs. 1-5). Following enlargement of the projection, the body of zygote gradually wrinkled and shrank and became smaller (Figs. 2-5). An annular structure developed between the newly formed ookinete and the remaining body of zygote (Figs. 5-6), which ultimately separated from the ookinete.Tile ookinete was banana-shaped. The surface of mature ookinetes was smooth (Fig. 13). On the tip there was an apical complex showing truncated cone shape. Behind the apical end there were a few pellicular folds (Figs. 9-11).Through the course of development of the ookinete the parasite constantly showed obvious movement. As the parasites protruding forward, longitudinal and spiral wrinkles appeared on their body surface (Figs. 2-4, 10-12).