Diets of bison (Bison bison L.) were examined using microhistological fecal analysis in a 2-yr study on a tallgrass prairie site in northcentral Oklahoma. Graminoids comprised at least 98% of the diet across all seasons. Bison showed strong feeding selectivity; grasses and sedges formed a significantly higher proportion of diets than was generally available in herbage on the landscape. Bison avoided forbs, which were less than or equal to 2% of the diet. Sedges were a large (17-44%) diet component in winter and spring but decreased substantially during summer and fall (11-16%). These changes in sedge use corresponded to seasonal variation in sedge availability. Our results confirm that bison are primarily grazers in prairie habitats, potentially having a significant role in shaping structure and function of tallgrass prairie.