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To analyze the stable isotope ratios of small‐bodied invertebrates, the entire animal is typically sacrificed and processed, which is problematic for threatened or endangered species. Appendages which are regenerated could be used to infer whole‐body isotope ratios, but differences in turnover rates and isotopic signatures among tissues may confound such an approach. We tested the hypothesis that the δ¹³C and δ¹⁵N of whole‐body tissue for freshwater amphipods could be predicted from the δ¹³C and δ¹⁵N of walking legs, with the goal of estimating body δ¹³C and δ¹⁵N of Gammarus acherondytes, a United States federally endangered species. To test this, we analyzed the δ¹³C and δ¹⁵N of walking legs and bodies of five species of amphipods from geographically distant areas (Idaho, Illinois, and Washington) in the United States. The general relationships of whole‐body isotope ratios of C and N as a …