To understand how the increase in atmospheric CO₂ from human activity may affect leaf damage by forest insects, we examined host plant preference and larval performance of a generalist herbivore, Antheraea polyphemus Cram., that consumed foliage developed under ambient or elevated CO₂. Larvae were fed leaves from Quercus alba L. and Quercus velutina Lam. grown under ambient or plus 200 μl/liter CO₂ using free air carbon dioxide enrichment (FACE). Lower digestibility of foliage, greater protein precipitation capacity in frass, and lower nitrogen concentration of larvae indicate that growth under elevated CO₂ reduced the food quality of oak leaves for caterpillars. Consuming leaves of either oak species grown under elevated CO₂ slowed the rate of development of A. polyphemus larvae. When given a choice, A. polyphemus larvae preferred Q. velutina leaves grown under ambient CO₂; feeding on foliage of this species grown under elevated CO₂ led to reduced consumption, slower growth, and greater mortality. Larvae compensated for the lower digestibility of Q. alba leaves grown under elevated CO₂ by increasing the efficiency of conversion of ingested food into larval mass. Despite equivalent consumption rates, larvae grew larger when they consumed Q. alba leaves grown under elevated compared with ambient CO₂. Reduced consumption, slower growth rates, and increased mortality of insect larvae may explain lower total leaf damage observed previously in plots in this forest exposed to elevated CO₂. By subtly altering aspects of leaf chemistry, the ever-increasing concentration of CO₂ in the atmosphere will change the trophic dynamics in forest ecosystems.