Diets of prairie dogs, goats, and sheep were examined by microhistological fecal analysis during 4 periods of a year in a desert rangeland in northern Mexico. Prairie dogs selected more grasses (79% across all seasons; P < 0.05) than goats and sheep during most of the year. Total grasses in goat diets were consistent (20%) in all seasons, whereas this forage class was highest during winter (72%) and lowest during summer (62%) in sheep diets. The diet of goats was predominantly shrubs (45%–62%) in all seasons, whereas sheep and prairie dogs ate little browse throughout the study. All 3 species preferred forbs, which contributed about one-third to the composition of the prairie dog (winter), goat (summer), and sheep (spring) diets. Acacia greggii Gray and Opuntia rastrera Weber were the most preferred species by goats, whereas prairie dogs and sheep showed particular preference for Buchloe dactyloides (Nutt.) Engelm. and Bouteloua gracilis (Willd. ex Kunth) Lag. ex Griffiths. During summer and spring, concentration of nitrogen in the feces of sheep was 36% and 17% higher (P < 0.05) than in the feces of goats. There was a high overlap in diets between prairie dog and sheep in all seasons, whereas diets of prairie dogs and goats, and goats and sheep were significantly different from each other in all seasons. These results showed that competition was keen between prairie dogs and sheep for a limited quantity of forage in this arid zone pasture, whereas goats were better able to use common resources with prairie dogs.