Despite the widespread use of N,N,-diethyl-3-methylbenzamide (deet) in insect repellent products, nothing is known about the molecular basis for the repellency of deet. We initiated a molecular genetics program to elucidate the molecular mechanism of deet repellency in Drosophila melanogaster (Meigen). Deet repellency was apparently due to airborne vapors, as wild type flies were repelled by a deet-treated surface in the absence of physical contact and in the dark. A mutant was isolated using chemical mutagenesis and a choice assay. In a choice assay, mutant flies entered 82 ± 1% of deet-containing tubes, whereas wild type flies entered only 6 ± 2% of deet-containing tubes. The mutant was repelled by other repellents, benzaldehyde and citronellal. The mutation was recessive and located on the X chromosome.