none A sex pheromone of Ascogaster reticulatus WATANABE (Hymenoptera: Braconidae) which is deposited on the substrate by females and elicits male antennal-searching response, was analyzed and characterized. The hexane extract of the virgin females was subjected to silica gel column chromatography. An active fraction (active at 3.3 times 10-1-10-2 ng/line) was analyzed with gas chromatography (GLC) and GC-MS, and three major peaks were identified as citronellal (44%), hexadecanal (12%) and hexadecenal (43%). Double bond location at ninth position in hexadecenal was determined by GC-MS after dimethyl disulfide derivitization. Geometric isomers of 9-hexadecenal were both co-chromatographed on GLC and (Z)-9-hexadecenal was identical to the natural 9-hexadecenal. Bioassay with these authentic compounds showed that only (Z)-9-hexadecenal elicited pheromonal activity (active at 5 times 10-4-5 ng/line). Therefore, (Z)-9-hexadecenal was identified as the sex pheromone of A. reticulatus.