Bighorn sheep (Ovis c. canadensis) were intensively studied on the northern Yellowstone winter range duirng the winter of 1965-66, with additional data gathered the following two winters. Censuses showed a stable or slowly increasing population of about 250 animals. Grass and browse utilization was moderately high. During the winter of 1965-66, bighorns ate 61 percent grass, 17 percent forbs, and 22 percent browse. Bluebunch wheat grass (Agropyron spicatum) was the most common species in the diet. Snow cover was light that winter, and competition with elk (Cervus canadensis) was slight to moderate.