The entry of sporozoites of Theileria parva into bovine lymphoid cells in vitro was studied with the electron microscope. Endocytosis is completed in less than 10 min. No local mobilization of actin or other cytoskeletal elements is detected in the cytoplasm of the cell being invaded and no engulfing pseudopods are formed. At the site of initial contact, the membranes of parasite and host cell come into very close apposition. As the zippering up of the membranes spreads laterally, the sporozoite sinks into a progressively deepening recess in the surface of the host cell until the rim of the invagination closes and fuses over the parasite. The observation that sporozoites are interiorized at 1-2 degrees C as well as at 37 degrees C suggests that endocytosis depends mainly upon a ligand-receptor interaction of the parasite and host cell membranes and requires little energy. Sporozoites may enter in any orientation, unlike other sporozoan parasites in which the membrane overlying an apical complex is invariably the site of attachment. 24 h after entry, the sporozoite is located in the Golgi region and the investing host cell membrane acquired during endocytosis has disappeared. The Golgi complex has been activated to form small lysosomes which gather around the parasite but are ineffective for lack of a membrane which they can fuse. It is suggested that removal of the investing host-cell membrane permits the parasite to evade destruction by the phagolysosomal system of the host cell. Persistence of micronemes after entry of the sporozoite and their subsequent disappearances invites the speculation that these parasite organelles may play a role in dispersal of the invaginated host cell membrane.