Elevated atmospheric concentrations of CO₂ and O₃ are known to alter the chemical composition of foliage, which in turn may affect the performance of herbivorous insects. We investigated the independent and interactive effects of CO₂ and O₃ on foliar quality of paper birch (Betula papyrifera Marshall) and the consequences of chemical changes for performance of the whitemarked tussock moth Orgyia leucostigma (J. E. Smith). The experimental design was a 2 by 2 factorial, with ambient and elevated levels of CO₂ and O₃, respectively. Foliage was analyzed for concentrations of nitrogen, starch, and condensed tannins. CO₂ and O₃ independently and interactively affected nitrogen concentrations, with the elevated CO₂ O₃ treatment reducing nitrogen concentrations more than either treatment alone. Elevated CO₂ and O₃ had no significant effect on starch and tannin concentrations when administered alone but increased starch concentrations by 17% over ambient when administered together. Larvae were reared on experimental trees from egg hatch through pupation to determine treatment effects on development time and pupal mass. Larval performance measures were not statistically different among fumigation treatments, although females tended to have reduced pupal mass under the elevated CO₂ O₃ treatment. These results demonstrate that chemical responses of some plant species to elevated levels of CO₂ (560 μl L⁻¹) and O₃ (1.5 × ambient) may be of insufficient magnitude to significantly alter standard measures of individual insect performance.