Benthic macroinvertebrate families were sampled along 3 rivers in New Brunswick, Canada. Stable isotopes of C and N were compared between body tissue and gut contents of individuals. δ¹³C and δ¹⁵N of body tissue and gut contents were strongly correlated (r = 0.94 and 0.93, respectively) over a wide range of δ values. In nonpredators, only minor fractionation of δ¹³C and δ¹⁵N was observed. In predators, diet–tissue fractionation of ¹³C was minor, but ¹⁵N fractionation that may have been related to diet quality (N content) was observed. The influence of diet quality on N-isotope fractionation was inconsistent in direction and strength among families. Our results suggest that subjecting primary consumers to gut clearance prior to processing for stable-isotope analysis is unnecessary, but the guts of predators should be removed before processing.