The skull of a mountain sheep (Ovis canadensis cremnobates) exhibiting osteoporosis was recovered from San Diego County, California. This specimen also exhibited lateral asymmetry of the cranium and rostrum, malocclusion of molars, and mandibular asymmetry. Investigators are cautioned about potentially confusing lesions associated with chronic frontal sinusitis with lesions of osteoporosis in mountain sheep. To our knowledge this is the first report of osteoporosis in this species.