The endogenous and exogenous stages of a species of murine Eimeria were examined and compared with descriptions of other species isolated from Mus musculus. Developing stages of the parasite were found in the epithelial cells of the caecum and colon. Three asexual generations were observed, the third becoming apparent on the fourth day concurrently with gametogeny. The total length of the prepatent period was 5 days. Oocysts of the coccidian, identified as Eimeria falciformis, were administered to a group of germfree mice. Despite a dose level of 1 x 10(6), the coccidium was not established. Histological examination indicated the failure of the sporozoite to penetrate its host epithelium. The possible causes of this phenomenon were discussed.