Mysis relicta is a species of omnivorous zooplankton that has been implicated over the last several decades in important food-web processes such as the lengthening of food chains, the mobilization of benthic contaminants to the pelagic, and the restructuring of community dynamics. We used stable nitrogen isotopes ( d¹⁵N) to quantify ontogenetic diet shifting of M. relicta in five North American lakes. Analyses indicated that M. relicta measuring >1.5 cm were elevated, on average, 0.73 trophic levels above conspecifics measuring <1-cm length, and 0.57 trophic levels above conspecifics measuring >1 but <1.5-cm length. This pattern suggests a trend toward increasing carnivory with maturity, which is consistent with the results of previous conventional approaches to diet analysis in this species. Our results should be useful to modelers concerned with quantifying the effects of M. relicta on energy transfer and contaminant bioaccumulation in food chains.