The sex attraction responses of male populations of European corn borer,Ostrinia nubilalis(Hübner), to 5 ratios of theZ:Eisomers (1:0,97:3,1:1,3:97, and 0:1) of the sex pheromone, 11-tetradecenyl acetate, were monitored at 31 geographic locations in North America and Europe. The pattern of male response to the isomeric blends varied from one location to another. Most male populations on the 2 continents responded preferentially to the 97:3Z:Eblend, but populations at several locations were either attracted preferentially to 3:97Z:Eor showed heterogenity in response. Heterogeneity of male response was greatest in regions where the 2 pheromonal types of European corn borers occurred sympatrically.