We related the botanical composition and nutritional quality of the diets of 5 tame elk (Cervus elaphus nelsoni) to forage quality, advancing season, and plant communities of upper montane winter range during 1976-78. Graminoids dominated diets (x̄ = 61%) during 1976-77, but declined in amount from 66% of observed bites in November to 44% in February as browse consumption increased. Browse and grass (accounting for >90% of bites each month) contributed equally to diets through winter 1977-78. Browse species contained more crude protein, cell solubles, and lignin, and less cellulose and in vitro digestible dry matter (IVDDM) than grasses. Nutritional quality of browse remained constant with advancing season, whereas grasses decreased in IVDDM and crude protein, and increased in fiber content. Grasses and browse contained less crude protein, and browse less IVDDM, during 1977-78 than during the previous year. Diet quality was influenced by changes in forage quality; during November-March 1976-77, dietary crude protein declined from 5.8 to 4.9%, and IVDDM declined from 49 to 40%. Over the same period in the following year, dietary crude protein declined from 5.4 to 4.6%; IVDDM from 42 to 37%. During 1976-77, mean overwinter dietary protein ranged from 6.3% in dry grassland to 4.6% in mesic meadow communities, whereas dietary IVDDM ranged from 47% in aspen to 35% in grasslands. During 1977-78, protein was highest (5.6%) in willow and lowest (4.6%) in mesic meadow, and IVDDM was highest (39%) in aspen and lowest (35%) in willow. Despite year-to-year variation in forage quality, elk maintained relatively stable diet quality over time and space by shifting the forage-class mix of diets.