Identifying the source of infectious disease outbreaks is difficult, especially for pathogens that infect multiple wildlife species. Brucella spp. are among the most problematic zoonotic agents worldwide, and they are notoriously difficult to detect and identify. We genotyped 10 variable number of tandem repeat (VNTR) DNA loci in 56 Brucella abortus isolates from bison (Bos bison), elk (Cervus elaphus), and cattle (Bos taurus) to test the wildlife species most likely to be the origin of recent outbreaks of brucellosis in cattle in the Greater Yellowstone Area. Isolates from cattle and elk were nearly identical but highly divergent from bison isolates. These data suggest elk, not bison, are the reservoir species of origin for these cattle infections. This study illustrates the potential power of VNTR genotyping to assess the origin of disease outbreaks, which are increasing worldwide following habitat fragmentation, climate change, and expansion of human and livestock populations.