Beet armyworm,Spodoptera exigua(Hübner), larvae were placed on sugarbeet (Beta vulgarisL.) and pigweed (Amaranthus hybridusL.) plants in outdoor chambers in which the plants were growing at either the ambient (≈350μl liter−1) or ambient plus 350μl liter−1(≈700μl liter−1) carbon dioxide concentration. A series of experiments was performed to determine if larvae reduced plant growth differently at the two carbon dioxide concentrations in either species and if the insect growth or survival differed with carbon dioxide concentration. Leaf nitrogen, water, starch, and soluble carbohydrate contents were measured to assess carbon dioxide concentration effects on leaf quality. Insect feeding significantly reduced plant growth in sugarbeet plants at 350μl liter−1but not at 700μl liter−1nor in pigweed at either carbon dioxide concentration. Larval survival was greater on sugarbeet plants at the elevated carbon dioxide concentration. Increased survival occurred only if the insects were at the elevated carbon dioxide concentration and consumed leaf material grown at the elevated concentration. Leaf quality was only marginally affected by growth at elevated carbon dioxide concentration in these experiments. The results indicate that in designing experiments to predict effects of elevated atmospheric carbon dioxide concentrations on plant–insect interactions, both plants and insects should be exposed to the experimental carbon dioxide concentrations, as well as to as realistic environmental conditions as possible.