In an effort to genetically dissect the olfactory response in Drosophila melanogaster, we screened for mutants defective in their response to propionic acid in an olfactory jump assay. Nine mutants were isolated following P element transposition. The defects were found to be specific to olfaction, as the mutants showed apparently normal response to a number of other sensory stimulations. All mutations were recessive and X-linked. Meiotic recombination mapping and testing of established recombinant lines showed that seven of the mutations can be mapped to a ingle chromosomal region, suggesting that the defects are due to single gene mutations. These mutants were characterized with respect to their response to several odorants by the jump assay and by electrophysiological recording. Seven mutants showed a reduced response to propionic acid and ethyl acetate (both attractants), but normal to benzaldehyde (a repellent), hence may define separate signaling pathways for attractants and repellents. Two mutants are defective to three different odorants, hence may affect a step shared by these separate pathways. A few of the mutants have defective electroantennogram (EAG) response, suggesting that the defect may be at a peripheral step affecting sensory reception and/or transmission. Others have an apparently normal EAG response, suggesting that the defect may be at more central steps.