Presumably due to their association with nitrogen-fixing bacteria, the nutritional quality of legumes decreases less than that of non-legume C3 plants when grown under elevated atmospheric CO2. Therefore, it seems likely that legume-feeding herbivores will be less adversely affected than herbivores of non-legume C3 plants by anthropogenic increases in atmospheric CO2. When the legumes Medicago sativa (alfalfa), Trifolium repens (white clover), and Lotus corniculatus (birdsfoot trefoil) were grown under elevated (756 ppm) CO2, leaf nitrogen remained the same or increased, and C:N ratio did not change. Unlike most insects fed non-legume C3 plants, Colias philodice (sulfur butterfly) larvae fed elevated-grown M. sativa and T. repens did not exhibit reduced relative growth rate (RGR), and larvae fed elevated-grown L. corniculatus exhibited a nearly significant 37% increase in RGR. Pupal weight was unaffected by growth of host plants under elevated CO2. Relative nitrogen growth rate (RGRN) did not change for larvae fed elevated-grown M. sativa or T. repens, but increased by 34% for larvae fed elevated-grown L. corniculatus. These results suggest that legume-feeding herbivores will be relatively buffered against the adverse effects of elevated CO2 typically experienced by herbivores of non-legume C3 plants.