Sporozoites of Eimeria vermiformis from the mouse were first seen in the epithelial cells of villus tips and the crypts of Lieberkühn four hours after inoculation (HAI). They were always within a parasitophorous vacuole. By 12 HAI, most were in crypt epithelial cells between the basement membrane and host cell nucleus. The sporozoites in the villus tips had 26 subpellicular microtubules, two polar rings, two preconoidal rings, two refractile bodies surrounded by amylopectin-like granules, a lamellar Golgi apparatus, numerous micronemes, and rhoptries. The sporozoites in the crypt cells had fewer amylopectin-like granules, micronemes, and rhoptries. A nucleolus was visible, as were pieces broken off from the posterior refractile body. Later, the sporozoites folded over to become U-shaped; the infolded membranes fused; and then the inner membranes disappeared so that spherical meronts were formed. Folding sporozoites were first seen 16 HAI and persisted until 52 HAI.