Interior cutthroat trout occupy small fractions of their historic ranges and existing populations often are relegated to headwater habitats. Conservation requires balancing protection for isolated genetically pure populations with restoration of migratory life histories by reconnecting corridors between headwater and mainstem habitats. Identification of alternative life history strategies within a population is critical to these efforts. We tested the application of nitrogen stable isotopes to discern fluvial from resident Bonneville cutthroat trout (BCT; Oncorhynchus clarkii utah) in a headwater stream. Fluvial BCT migrate from headwater streams with good water quality to mainstem habitats with impaired water quality. Resident BCT remain in headwater streams. We tested two predictions: (i) fluvial BCT have a higher ??¹⁵N than residents, and (ii) fluvial BCT ??¹⁵N reflects diet and ??¹⁵N enrichment characteristics of mainstem habitats. We found that fluvial ??¹⁵N was greater than resident ??¹⁵N and that ??¹⁵N was a better predictor of life history than fish size. Our data also showed that fluvial and resident BCT had high diet overlap in headwater sites and that ??¹⁵N of lower trophic levels was greater in mainstem sites than in headwater sites. We conclude that the high ??¹⁵N values of fluvial BCT were acquired in mainstem sites.