The sex attractant of Porthetria dispar (L.), called disparlure, was isolated from a crude fraction of 78,000 abdominal tips of the female moth. Activity of the fraction was monitored by both field and laboratory bioassay. After partial purification of the crude fraction by saponilication and column and thin-layer chromatography (TLC), the attractant was characterized by thin-layer and gas chromatographic tests as an epoxide with 18-20 carbon atoms. Because insufficient attractant was available, all attempt to find and isolate an olefin precursor of the epoxide was made. and it proved successful. The olefin was identified by microozonolysis, TLC, micro hydrogenation, and mass spectral analysis as 2-methyl-cis-7-octadecene. The sex attractant is therefore cis7,8-epoxy-2-methyloctadccane. It was synthesized in 4 steps with an overall yield of 60%. The synthetic compounds were identical to the natural precursor and sex lurc by combined gas chromatography-mass spectrometry. The sex attractant was very active in field tests. Eight compounds closely related to disparlure were synthesized, but they were far less active than disparlure.