To determine whether the rhoptries of Toxoplasma gondii play a role in the invasion of host cells by this parasite, we inoculated toxoplasmas into the peritoneal cavities of normal mice and into macrophage cultures, fixed the specimens at various intervals thereafter, and analyzed them by electron microscopy. We found that during host-cell invasion, the rhoptry membrane fused with the anterior limiting membrane of the toxoplasma, producing an opening to the exterior. Since such openings were formed when the host-cell membrane was disrupted, it appears that the rhoptries may secrete a lytic product that facilitates invasion through the host-cell membrane. Such a "penetration-enhancing factor" was previously isolated from lysed toxoplasmas (Lycke and Norrby, 1966). Occasionally, when secretion was incomplete, masses of tubules were found in the rhoptries, sometimes as soon as 15 sec after the toxoplasms had been injected into mice. Similar tubules were found in the parasitophorous vacuole that was formed 10-15 min later, and such tubules are typical of vacuoles containing replicating parasites. Because these tubules are in continuity with the vacuolar membrane, it appears to be a hybrid membrane, composed in part of toxoplasma products. We speculate that the hybrid nature of the vacuolar membrane prevents it from fusing with the lysosomes of phagocytes and thereby contributes to the intracellular survival of the parasites.