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.