Fire and native large herbivore grazing were two important influences on the structure and function of North American grasslands. In 1988 and 1989 the influence of fire regime on grazing patterns of North American bison (Bison bison) was studied on the Konza Prairie in northeastern Kansas. Bison grazing was spatially and temporally nonrandom and was influenced by fire regime and local plant community composition. During the growing season, bison were observed up to 3 x more frequently than expected on watersheds burned in the spring. Summer grazing was concentrated in large watershed areas (79-119 ha) dominated by warm-season, perennial, C4 grasses. During the autumn and winter, bison grazed both burned and unburned watersheds more uniformly but grazed most intensively in areas with large stands of cool-season, C3 grasses. On a smaller spatial scale (5-10 m2), bison selected patches during the growing season with low forb cover dominated by the perennial C4 grass, Andropogon gerardii. Grazed patches were larger on frequently burned than on infrequently burned watersheds. The importance of fire history in determining patterns of bison grazing over the landscape indicates that interactions between bison grazing and fire regime may be important to the composition and spatial heterogeneity of tallgrass prairie vegetation.