A water-extractable host recognition kairomone in frass of corn earworm,Helicoverpa zea Boddie (Lepidoptera: Noctuidae), host larvae stimulates antennation by females of the parasitoidMicroplitis croceipes Cresson (Braconidae: Hymenoptera). In addition, when the wasps contact water extracts of host frass they will subsequently fly in a flight tunnel to odor associated with the extract. Contact with water extracts of cowpea leaves or with water extracts of frass from larvae of nonhost beet armyworm, fall armyworm, or cabbage looper that were fed cowpea leaves does not stimulate antennation, nor do wasps fly to associated odors after contact with these substances. However, contact with the water extract of host frass in association with hexane extract of cowpea-fed nonhost frass will induce the wasps to subsequently fly to the hexane extract of the nonhost frass when it is used as an odor source in a flight tunnel. Thus the host-specific kairomone by whichM. croceipes recognizes the frass of its host is extractable with water. This substance plays a crucial role in the foraging behavior of this parasitoid by allowing it to recognize host frass and to learn to search for odors originating from plants on which the host is feeding.