By capillary gas chromatography, mass spectrometry, electroantennogram measurements, and male behavioral response tests, (Z)-5,(E)-7-dodecadienal was identified as the principal component of the sex pheromone of the forest tent caterpillar,Malacosoma disstria(Hubner). (Z)-5,(E)-7-Dodecadien-l-ol was indicated as a secondary component. In field trapping males were attracted to lures containing each component separately but best trapping occurred with aldehyde:alcohol ratios between 1:10 and 1:3.n-Dodecanal was also found in female tip extracts.