In North America, the role of wild ruminants in the epidemiology of anaplasmosis has not been clearly defined. Such information is particularly meager in regard to bighorn sheep. We report the susceptibility of two Rocky Mountain bighorn sheep (Ovis canadensis canadensis) to experimental infection with a well characterized field isolate of Anaplasma ovis obtained from domestic sheep in Idaho. Both bighorn sheep developed infection resulting in severe clinical disease, with relatively high parasitemias, icterus and anemia. One animal required tetracycline therapy and responded well to treatment, while the other recovered uneventfully without treatment. Both bighorn sheep were spleen-intact, a condition that in A. ovis-exposed domestic sheep typically is associated with mild infection. The results indicate that bighorn sheep may be adversely affected if exposed to the organism in nature.