The olfactory responses ofPeristenus pseudopallipes(Loan) to the odors of some food plants of its host, the tarnished plant bug (Lygus lineolarisPalisot deBeauvois), were studied. The parasitoid females responded preferentially toErigeronplant odor in a Y-tube olfactometer. The attractive odor was shown to emanate from the flowers. The responsiveness of theP. pseudopallipesfemales was shown to be related to the number of eggs in their ovaries. The flowers ofErigeronwere also preferred for feeding, and this feeding apparently was necessary for maximum longevity.