The sex pheromone of the calendula plume moth,Platyptilia williamsii was identified as (Z)-11-hexadecenal (Z11-16∶Aid). Extracts of female sex pheromone glands contained several compounds when analyzed by capillary and packed-column GLC. However, airborne collections of volatiles from glands contained only one of these compounds, having the same retention time asZ11-16∶Ald. GC-MS and microozonolysis analyses of the natural product were consistent with those of syntheticZ11-16∶Ald. In a flight tunnel, males oriented upwind and touched sources ofZ11-16∶Ald and gland extract with equal frequency. Field tests of syntheticZ11-16∶Ald already have shown it to be a potent sex attractant for males of this species. This study further supports the hypothesis thatP. williamsii and a sympatric species,Platyptilia carduidactyla, are not reproductively isolated by chemical differences in the composition of the sex pheromone, but rather by temporal differences in sexual activities.