Black-footed ferrets are predators and feed almost exclusively on prairie dogs, although prairie-dog predation does not seem to significantly reduce the size of prairie dog populations [5]. The precipitous decline of black-footed ferrets is largely associated with virtual elimination of their prey. Between 1900 and 1960 government programs and private efforts at prairie dog control eliminated prairie dogs from about 98% of their range [4]. Black-footed ferrets are also very sensitive to canine distemper and to sylvatic plague. All six individuals removed from the Meeteetsee population for captive breeding in 1985 died from canine distemper, and the Meeteetse population declined from about 170 in 1984 to about 16 in September 1985 after sylvatic plague was found in prairie dogs at the site.