Presumably due to their association with nitrogen-fixing bacteria, the nutritional quality of legumes decreases less than that of non-legume C 3 plants when grown under elevated atmospheric CO 2 . Therefore, it seems likely that legume-feeding herbivores will be less adversely affected than herbivores of non-legume C 3 plants by anthropogenic increases in atmospheric CO 2 . When the legumes Medicago sativa (alfalfa), Trifolium repens (white clover), and Lotus corniculatus (birdsfoot trefoil) were grown under elevated (756 ppm) CO 2 , leaf nitrogen remained the same or increased, and C:N ratio did not change. Unlike most insects fed non-legume C 3 plants, Colias philodice (sulfur butterfly) larvae fed elevatedgrown 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 CO 2 . 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 CO 2 typically experienced by herbivores of non-legume C 3 plants.