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Bison (Bison bison) are a keystone species in the North American Great Plains. They are also a species of conservation concern in part because they suffered a severe population bottleneck at the end of the 19th century and now exist in mostly small and isolated populations. Wind Cave National Park introduced 20 bison in 1913–16; Theodore Roosevelt National Park introduced 29 animals to a South Unit in 1956 and subsequently transferred 20 bison from that herd to the park's North Unit in 1962; and Badlands National Park introduced 53 animals in 1963–64 and another 20 in 1984. The four herds are confined within fences, and no known introduction of new bison into the gene pools has occurred. The four herds are routinely culled down to approximately 425, 350, 200, and 700 animals, respectively. I found no evidence of inbreeding depression as measured by annual population growth and reproductive rates. A population viability analysis also failed to find persuasive evidence of inbreeding depression. Under current management scenarios, gene diversity in 100 years should remain above levels currently known to cause inbreeding depression in bison. I found no compelling reason to introduce new bison into the herds or transfer bison between herds in the foreseeable future.