Productivity and performance were evaluated over a 4-year period for a herd of nonsupplemented bison (Bos bison) at the Konza Prairie Research Natural Area in northeastern Kansas. Calving rates ranged from 65.7% to 79.2%, with an average of 74.4%. Out of 317 calves born, 47.3% were male and 52.7% were female. Numbers of male and female calves born each year did not deviate from an expected 50:50 ratio except in 1997. At 6 months of age, male bison calves were significantly heavier than female calves. By 7.5 years of age, females weighed an average of 455 kg, and males averaged 727 kg. Male and female calves gained an average of 2.6% of their body mass over their first winter, but all other animals lost an average of 11.3% of their body mass over winter. All winters were relatively mild during this study, and mass losses for different age and sex groups did not differ significantly among years. Performance and productivity data for the Konza Prairie bison provide a benchmark for comparison with other herds.