Grizzly bears (Ursus arctos) cohabit landscapes with a diversity of ungulates, but the degree to which grizzly bears are carnivorous is unclear and likely varies across landscapes. We used stable isotopes of carbon and nitrogen to infer seasonal diets of grizzly bears in northern British Columbia while studying predator–prey dynamics in a largely undisturbed system. We found high seasonal variation in diets among individuals; males consumed more ungulate prey than did females throughout the year. In fall, both sexes increased their consumption of prey; large ungulates constituted 51% and 32% of the fall diets of males and females, respectively. This increase in carnivory appeared to be primarily associated with consumption of elk (Cervus elaphus). Estimates of prey proportions in the diets of grizzly bears were highly sensitive to the range of tissue to diet discrimination values that were incorporated in isotope models. Small changes in discrimination values resulted in estimated prey intakes that varied more than threefold as a percentage of the diet depending on sex and season. We caution against using standard discrimination values, and we recommend that diet reconstructions using stable isotopes be based on tissue-specific values that would be appropriate for the species of interest. © 2012 The Wildlife Society.