Between 1979 and 1980, 104 bats representing 13 species in 4 families were collected in California and New Mexico, U.S.A., and Baja California and Sonora, Mexico, and were examined for coccidia; only 3 (3%) had oocysts in their feces. Bats examined and their infection rates were: Molossidae: 0 of 12 Tadarida brasiliensis, 1 of 18 (6%) T. femorosacca; Natalidae: 0 of 1 Natalus stramineus; Phyllostomatidae: 0 of 1 Choeronycteris mexicana, 0 of 2 Leptonycteris sanborni, 0 of 1 Macrotus californicus; Vespertilionidae: 0 of 9 Antrozous pallidus, 0 of 28 Eptesicus fuscus, 0 of 1 Lasionycteris noctivagans, 0 of 3 Lasiurus borealis, 2 of 22 (9%) L. cinereus, 0 of 1 L. ega, 0 of 5 Pipistrellus hesperus. Sporulated oocysts were only found in T. femorosacca and these represent a new species, Eimeria tadarida n. sp. They are subspheroidal to ellipsoidal, 19 x 25 (16-23 x 20-30) microns; a micropyle is absent, and fragments within the oocyst may be oocyst residuum or multiple polar bodies. The oocyst wall, approximately 1.5 microns, is composed of a mammillated outer layer and smooth inner layer. Sporocysts are ovoidal, 8 x 12 (6-9 x 10-14) microns, and have a small Stieda body and a wide substieda body. This is only the 14th eimerian to be described from bats worldwide. Only unsporulated or partially sporulated oocysts of an eimerian were seen in 2 L. cinereus. These measured 28 x 25 (27-29 x 24-26) microns and had a mammillated outer oocyst wall.