It is plausible that the nutritional quality of C3 plants will decline more under elevated atmospheric CO2 than will the nutritional quality of C4 plants, causing herbivorous insects to increase their feeding on C3 plants relative to C4 plants. We tested this hypothesis with a C3 and C4 grass and two caterpillar species with different diet breadths. Lolium multiflorum (C3) and Bouteloua curtipendula (C4) were grown in outdoor open top chambers at ambient (370 ppm) or elevated (740 ppm) CO2. Bioassays compared the performance and digestive efficiencies of Pseudaletia unipuncta (a grass-specialist noctuid) and Spodoptera frugiperla (a generalist noctuid). As expected, the nutritional quality of L. multiflorum changes to a greater extent that did that of B. curtipendula when grown in elevated CO2; levels of protein (considered growth limiting) declined in the C3 grass, while levels of carbohydrates (sugar, starch and fructan) increased. However, neither insect species increased its feeding rate on the C3 grass to compensate for its lower nutritional quality when grown in an elevated CO2 atmosphere. Consumption rates of P. unipuncta and S. frugiperda were higher on the C3 grass than the C4 grass, the opposite of the result expected for a compensatory response to the lower nutritional quality of the C4 grass. Although our results do not support the hypothesis that grass-specialist insects compensate for lower nutritional quality by increasing their consumption rates more than do generalist insects, the performance of the specialist was greater than that of the generalist on each grass species and at both CO2 levels. Mechanisms other than compensatory feeding, such as increased nutrient assimilation efficiency, appear to determine the relative performance of these herbivores. Our results also provide further evidence against the hypothesis that C4 grases would be avoided by insect herbivores because a large fraction of their nutrients is unavailable to herbivores. Instead, our results are consistent with the hypothesis that C4 grasses are poorer host plants primarily because of their lower nutrient levels, higher fiber levels, and greater toughness.