Increasing atmospheric concentrations of ozone and CO2 affect many aspects of tree physiology. However, their effects on tree resistance to insects have received relatively little attention. The objectives of this study were to test the effects of elevated CO2 and ozone on the resistance of three quaking aspen (Populus tremuloides) clones (216, 259, and 271) to first and fourth instars of four Lepidoptera species: gypsy moth (Lymantria dispar), forest tent caterpillar (Malacosoma disstria), large aspen tortix (Choristoneura conflictana), and whitemarked tussock moth (Orgyia leucostigma). Larval survival, growth rates, and nutritional indices were quantified. There were no treatment effects on larval survival. Elevated CO2 decreased the growth rates of both instars of all species, except that of first instar forest tent caterpillar on aspen clone 216, which was increased. Elevated ozone increased the growth of first and fourth instars of all insect species tested. The treatment effects on growth rate were generally caused by their effects on the ability of larvae to convert digested food to biomass (ECD). Elevated ozone increased ECD. The effects of elevated CO2 on ECD were clone dependent: elevated CO2 decreased ECD on clones 271 and 259, but increased ECD on clone 216. Ozone had no effect on larval cosumption rates. Elevated CO2 decreased the consumption rate of large aspen tortrix but had no effect on the other species. This contrasts with other studies, in which elevated CO2 generally increased insect consumption. There were no statistically significant interactions between the CO2 and ozone treatments for any of the variables measured.