Active components of the sex pheromone of the peach fruit moth, Carposina niponensis WALSINGHAM, have been isolated and identified as (Z)-7-eicosen-11-one and (Z) or (E)-7-nonadecen-11-one. Therefore, sex-pheromone activities of 7-alken-11-ones of carbon numbers 17∼23 were evaluated with an electroantennogram (EAG), field trapping, and laboratory tests of sexual stimulation. On the EAG, the male antenna of peach fruit moth was more sensitive to each (Z)-isomer than to the corresponding (E)-isomer. Among the 12 homologues, (Z)-7-eicosen-11-one wasconfirmed to be most active, followed by (Z)-7-nonadecen-11-one. On field trapping, the activity as attractant for males was confirmed only for (Z)-isomers of 17∼20 carbon homologues. Of these, (Z)-7-eicosen-11-one (A) and (Z)-7-nonadecen-11-one (B) were conspicuously attractive. Furthermore, the activity was synergistically enhanced by mixing the two compounds at the ratio of 20:1 for A to B. This synergistic action of the two compounds was also confirmed in sexual stimulation under the laboratory conditions. These results confirm that (Z)-7-eicosen-11-one and (Z)-7-nonadecen-11-one are the major and the minor components, respectively, of the female sex pheromone of the peach fruit moth.