Although the Drosophila visual system has been described extensively, little is known about its olfactory system. A major reason for this discrepancy has been the lack of simple, reliable means of measuring response to airborne chemicals. This paper describes a jump response elicited by exposing Drosophila to chemical vapors. This behavior provides the basis for a single-fly chemosensory assay. The behavior exhibits dose dependence and chemical specificity: it is stimulated by exposure to ethyl acetate, benzaldehyde, and propionic acid but not ethanol. Animals can respond repeatedly at short intervals to ethyl acetate and propionic acid. The response relies on the third antennal segments. To illustrate the use of this behavior in genetic analysis of chemosensory response, nine acj mutants defective in response are isolated (acj = abnormal chemosensory jump), and their responses to two chemicals are characterized. All of the acj mutants are normal in giant fiber system physiology, and two exhibit defects in visual system physiology.