Four Eimeria species have been reported from American bison (Bison bison): E. auburnensis, E. bovis, E. brasiliensis, and E. canadensis. We report on finding two additional species: E. ellipsoidalis and E. zuernii. In 1993, the prevalence of these six species in 31 yearling bison in a recently-established herd on private ranchland in Montana (USA) was: E. bovis, 97%; E. canadensis, 90%; E. zuernii, 32%; E. ellipsoidalis, 32%; E. auburnensis, 9.7%; and E. brasiliensis, 3.2%. Eimerian oocysts also were recovered from five (14%) of 36 bison in 1993 from West Yellowstone; the species involved were: E. canadensis, E. bovis, E. zuernii and E. auburnensis.