We investigated the effects of host species and resource (carbon dioxide, nitrate) availability on activity of detoxication enzymes in the gypsy moth,Lymantria dispar. Larvae were fed foliage from quaking aspen or sugar maple grown under ambient or elevated atmospheric CO2, with low or high soil NO 3 (-) availability. Enzyme solutions were prepared from larval midguts and assayed for activity of cytochrome P-450 monooxygenase, esterase, glutathione transferase, and carbonyl reductase enzymes. Activity of each enzyme system was influenced by larval host species, CO2 or NO 3 (-) availability, or an interaction of factors. Activity of all but glutathione transferases was highest in larvae reared on aspen. Elevated atmospheric CO2 promoted all but transferase activity in larvae reared on aspen, but had little if any impact on enzyme activities of larvae reared on maple. High NO 3 (-) availability enhanced activity of most enzyme systems in gypsy moths fed high CO2 foliage, but the effect was less consistent for insects fed ambient CO2 foliage. This research shows that gypsy moths respond biochemically not only to interspecific differences in host chemistry, but also to resource-mediated, intraspecific changes in host chemistry. Such responses are likely to be important for the dynamics of plantinsect interactions as they occur now and as they will be altered by global atmospheric changes in the future.