Fifty-day-old fry of tilapia hybrids (Oreochromis aureus x niloticus) were placed in aquaria containing sediment with oocysts of Eimeria (sensu lato) vanasi Landsberg et Paperna. In the first 29 h after exposure sporulated oocysts in the stomach and free sporozoites in the gut could be found in examined fish. By 7 to 56 h after exposure, sporozoites, with their characteristic crystalloid body, were detected in intraepithélial lymphocyte-like and other leucocyte-like cells, but never in the epithelial cells. Infected cells were confined to the epithelial layer and did not enter the lamina propria. Within this time, some of the sporo-zoitcs divided by endodyogeny, once or twice in succession, to form daughter sporozoites. The parent’s sporozoite crystalline body was divided between the offspring of the primary and secondary divisions.