Evaluating the factors that promote coexistence between ecologically similar species is crucial to understanding the evolution and assembly of herbivore communities. The Jarman–Bell principle presents a trade-off between diet quality and quantity as an axis for dietary niche segregation and has been suggested as a mechanism facilitating species coexistence. This idea holds that larger-bodied herbivores consume greater amounts of relatively low-quality plant resources, while smaller-bodied herbivores typically feed selectively on higher-quality resources. Most studies investigating the Jarman–Bell principle have examined free-living ungulates in African savannas. The diverse ungulate community in Yellowstone National Park, USA offers an opportunity to investigate the applicability of this principle in a temperate North American ecosystem. In this study we use fecal nitrogen (FN) and stable carbon isotope values (δ¹³C) to examine the relationship between body size and seasonal patterns of dietary niche segregation among five species of wild ungulates. Specifically, we test the predictions that: (1) diet quality decreases with increasing body mass, (2) interspecific differences in diet are greatest between the largest- and smallest-bodied species, and (3) smaller-bodied species have narrower dietary breadth than larger-bodied species. Diet quality, as indicated by digestibility, declined significantly with body mass, consistent with the empirical pattern predicted by the Jarman–Bell hypothesis. Significant interspecific differences in diet quality generally aligned with variation in body mass. When resources were limited during the winter, the relationship between body mass and diet quality was more pronounced, suggesting increased dietary niche segregation during the lean season. The results showed little evidence indicating that dietary breadth scaled allometrically with body mass, as the two species most similar in body mass displayed the greatest and least range of seasonal variation in both FN and δ¹³C. This study adds to the weight and breadth of evidence that diet quality is negatively correlated with body size in wild ungulate assemblages. Our findings underscore the importance of body size as a factor facilitating dietary niche segregation and promoting coexistence among ecologically similar ungulate species.