Electroantennograms were recorded from an herbivore, Pseudoplusia includens (Walker) (Lepidoptera: Noctuidae), and a parasitoid, Microplitis demolitor (Wilkinson) (Hymenoptera: Braconidae), exposed to 5- through 12-carbon aliphatic compounds of several chemical classes. The response of the herbivore was higher for the 6- and/or 7-carbon hydrocarbons, alcohols, aldehydes, esters, and ketones. The response of the parasitoid was higher for the 7- and 8-carbon hydrocarbons, aldehydes, and ketones. Responses of the herbivore and the parasitoid to alcohols were similar. Both the herbivore and the parasitoid were most sensitive to aldehydes and ketones, and least sensitive to alcohols and hydrocarbons. Responses of the parasitoid to hydrocarbons, aldehydes, and ketones were numerically higher than those of the herbivore. The adaptive significance of differential olfactory sensitivity between the herbivore and the natural enemy is discussed in relation to tritrophic interactions among plants, herbivores, and natural enemies. none