The endogenous life cycle of Eimeria caliginosa was studied in experimentally infected dusky rice rats, Melanomys caliginosus. All endogenous stages were located in the epithelial cells of villi in the small intestine. Both Giemsa-stained mucosal scrapings and histological sections were studied for the diagnosis of all the life-cycle stages. Eimeria caliginosa has 3 generations of meronts (M) that differ by size, shape, and number of merozoites (m), which also differ in their size, shape, and location of their nuclei within the cytoplasm of the meronts. The 3 meront types, M₁–M₃, respectively, had 20–33 (m₁), 5–9 (m₂), and 13–16 (m₃) merozoites. Macrogametocytes and microgametocytes, as well as macrogametes and microgametes, completed the sexual cycle, which concludes with the formation of unsporulated oocysts. This parasite's endogenous development produced severe intestinal lesions in the experimentally infected dusky rice rats. In our ongoing work to understand the biodiversity present in plants and animals of the protected Reserva Biológica Alberto Manuel Brenes (ReBAMB) field station in Costa Rica, we now have discovered 3 new Eimeria species, and this is the second complete life cycle in which we document both the asexual and sexual stages.