An ultrastructural study of numerous serial sections of Plasmodium chabaudi trophozoites at various growth stages, followed by 3-dimensional reconstruction, allowed us to describe more precisely the internalization process of the erythrocytic stroma, both in space and in time. Two endocytotic processes are apparent. (1) Pinocytosis — as soon as the merozoite has become a young trophozoite (ring stage), small double membrane-bound vesicles can be seen budding off around the whole periphery of the parasite. After the inner membrane of the vesicle has disappeared, the contents alter and a pigment crystal appears. (2) Cytostomal system — this phenomenon coexists with, and eventually replaces, pinocytosis. It consists of invagination of the membrane of the parasitophorous vacuole and of the plasmalemma, through a typical cytostome, in order to form a cytostomal vacuole. This extends into a long tube, the cytostomal tube, which eventually digitates. When the tube reaches its maximal size, the cytostome disappears and the tube remains open to the erythrocytic stroma by a simple aperture. A new cytostome can form elsewhere on the parasite surface and another tube can extend. Two or three such tubes can coexist in a trophozoite although only one cytostome is functional at one time. At the end of the tubes vesicles bud off, the contents of which become modified as described previously. The residual product of haemoglobin degradation is the malarial pigment.