Brucella abortus is the causative agent of brucellosis, a disease enzootic in populations of free-ranging elk (Cervus elaphus) and bison (Bison bison) in the Greater Yellowstone Ecosystem, USA. We define the distribution of the disease in elk throughout Wyoming, USA, using three epidemiologic/geographic classifications based on winter-foraging opportunity. Antibody prevalence for >3,300 yearling and adult, female elk, sampled from supplemental feedgrounds in western Wyoming, USA, since 1985, was 21.9%. Surveillance of nearly 3,800 winter free-ranging, adult, female elk, sampled from herd units adjacent to feedgrounds from 1991 to 2008, indicated a total antibody prevalence of 3.7%. In contrast, none of the 1,930 male and female elk sampled from 1991 to 2008 in herd units distant from feedgrounds in Wyoming, USA, demonstrated exposure to B. abortus. In comparison to elk, bison had a higher incidence of brucellosis. Surveillance of 321 cow bison harvested within the Absaroka and Jackson bison herds in western Wyoming, USA, from 1992 to 2008 yielded an antibody prevalence of 63.9%.