(9Z,11E)-Hexadeca-9,11-dienal (1) was identified as a major component of the sex pheromone of the sugarcane borer Diatraea saccharalis. The identification is based on mass spectral examination of the extracts prepared from calling females, derivatization experiments with 4-methyl-3,5-dihydro-1,2,4-triazole-3,5-dione (2), stereospecific synthesis of dienal 1, and gas chromatography (GC) with electroantennographic detection (GC-EAD). In GC analysis, the antennaly active component and synthetic dienal 1 show identical retention times on several GC phases. However, the behavioral activity of synthetic 1 was much weaker than that of natural extracts. This suggests that an additional component(s) might be involved in sexual communication of this species.