Yellow-bellied marmots (Marmota flaviventris) eat a wide variety of grasses, forbs, and seeds, but do not feed on all items in proportion to their abundance in the environment. In this study, relationships between the marmot diet and estimated protein, water, caloric value, relative biomass, and toxicity of the available plant species were investigated. The epidermis of all forb species did not survive digestive processes equally well; therefore, a quantitative analysis of the animals' diet based on fecal sampling was precluded. However, observations of where marmots feed and limited data from fetal analysis indicate that marmots chose their food plants on the basis of relative abundance, phenology, rejection or low ingestion of plant species with defensive compounds, nutritional quality, and energy requirements. Water content is unlikely to affect diet choice except during prolonged drought.