The Texas State Bison Herd is directly descended from the herd assembled from 5 wild-caught bison by Charles Goodnight in the 1880s. In 1997, 36 bison were used to establish a herd at Caprock Canyons State Park. To aid in the development of a long-term genetic conservation plan for this population, we examined and analyzed allelic variation at 54 microsatellite loci representing each of the nuclear chromosomes in the bison genome. The current Texas State Bison Herd population exhibits low genetic diversity and heterozygosity levels compared with bison at Yellowstone National Park and Theodore Roosevelt National Park. Parentage analysis indicates that relatively few adults have contributed offspring in the last 5 years, leading to low effective population size estimations and a rapid increase in the average age of animals in the herd. The very limited number of original founders, multiple population bottlenecks over the last 120 years, and chronically small population size, coupled with genetic drift and inbreeding, have resulted in dangerously low levels of genetic diversity. This, in turn, has likely triggered demographic problems such as low recruitment and high calf mortality rates. Population viability analysis based on current population demography reveals that there is a 99% chance of extinction of the herd within the next 41 years. Based on these findings, the continued existence of this historically important bison population appears doubtful without the introduction of new genetic variation from another plains bison herd.