When grown under elevated atmospheric carbon dioxide (CO2), leaf nitrogen content decreases less for legumes than for nonlegume C3 plants. Given that elevated CO2 adversely affects insect herbivores primarily through dilution of plant nitrogen, it is reasonable to expect that legume-feeding herbivores will be relatively buffered against CO2-induced reduction in performance. However, despite their ecological and economic importance, very few studies have addressed the effects of elevated CO2 on legumefeeding herbivores. Unlike the responses of the vast majority of nonlegume C3 plants, when the legumes Trifolium pratense and Melilotus alba were grown under elevated (742 ppm) CO2, leaf nitrogen and carbon contents and C:N ratios did not change. For Colias philodice larvae fed T. pratense, elevated CO2 had little or no effect on consumption, digestion, or conversion of whole food or nitrogen and, consequently, no effect on growth rate, instar duration, or pupal weight. For larvae fed M. alba, elevated CO2 had little or no effect on consumption of whole food or nitrogen, increased digestion but decreased conversion of both and, consequently, had no effect on growth rate, instar duration or pupal weight. These results suggest that, relative to herbivores of nonlegume C3 plants, legume-feeding herbivores will be less affected as atmospheric CO2 continues to rise.