Insect antennae have a primary function of detecting odors including sex pheromones and plant volatiles. The assumption that genes uniquely expressed in these antennae have an olfactory role has led to the identification of several genes that are integral components of odorant transduction. In the present study, differential display polymerase chain reaction (ddPCR) was used to isolate 25 antennal-specific mRNAs from the male sphinx moth Manduca sexta. Northern blot analyses revealed that one clone, designated G7-9, was antennal-specific and was highly enriched in male antennae relative to female antennae. In situ hybridization indicated that G7-9 expression was restricted to a spatial domain of the olfactory epithelium occupied exclusively by sex-pheromone-sensitive olfactory sensilla. Amino acid homology and phylogenetic analyses identified G7-9 as a glutathione-S-transferase (GST); we have named the full-length clone GST-msolf1. GSTs are known to function primarily in the detoxification of noxious compounds. Spectrophotometric and chromatographic analyses of total GST activity indicate that the endogenous GSTs of male and female antennae can modify trans-2-hexenal, a plant-derived green leaf aldehyde known to stimulate the olfactory system of M. sexta. The restricted localization of GST-msolf1 to sex-pheromone-sensitive sensilla, the fact that the sex pheromone of M. sexta consists of a complex mixture of aldehyde components, and the observation that antennal GSTs can modify an aldehyde odorant suggest that GST-msolf1 may have a role in signal termination. In the light of the more commonly observed role of GSTs in xenobiotic metabolism, we propose that GST-msolf1 may play a dual role of protecting the olfactory system from harmful xenobiotics and inactivating aldehyde odorants, especially components of the M. sexta sex pheromone.