Carbon (??13C) and nitrogen (??15N) isotopes in biological samples from large herbivores identify photosynthetic pathways (C3 vs. C4) of plants they consumed and can elucidate potential nutritional characteristics of dietary selection. Because large herbivores consume a diversity of forage types, ??13C and ??15N in their tissue can index ingested and assimilated diets through time. We assessed ??13C and ??15N in metabolically active liver tissue of sympatric mule deer (Odocoileus hemionus) and white-tailed deer (O. virginianus) to identify dietary disparity resulting from use of burned and unburned areas in a largely forested landscape. Interspecific variation in dietary disparity of deer was documented 2-3 years post-fire in response to lag-time effects of vegetative response to burning and seasonal (i.e., summer, winter) differences in forage type. Liver ??13C for mule deer were lower during winter and higher during summer 2 years post-fire on burned habitat compared to unburned habitat suggesting different forages were consumed by mule deer in response to fire. Liver ??15N for both species were higher on burned than unburned habitat during winter and summer suggesting deer consumed more nutritious forage on burned habitat during both seasons 2 and 3 years post-fire. Unlike traditional methods of dietary assessment that do not measure uptake of carbon and nitrogen from dietary components, analyses of stable isotopes in liver or similar tissue elucidated ??13C and ??15N assimilation from seasonal dietary components and resulting differences in the foraging ecology of sympatric species in response to fire.