Although aphids reproduce asexually on their host plants during the summer, many species migrate to a winter host where sexual reproduction occurs. Males of certain species locate mates by means of a sex attractant pheromone released from the hind legs of the females1–4. Here we report the identification of such a pheromone from the vetch aphid Megoura viciae. Extracts of the excised legs of sexual females were analysed by gas chromatography-coupled single cell recording techniques (GC-SCR)5 and by gas chromatography-coupled mass spectrometry (GC-MS). A tentative identification by MS analysis of the two active components as specific isomers of the monoterpenoids nepetalactone (I), a known cat attractant, and nepetalactol (II) was confirmed by comparison with authenticated compounds isolated or synthesized from natural products. Although neither compound was attractive to males when presented alone in the laboratory bioassay, a mixture of the two components produced a response equal to that elicited by the female leg extract. Thus the sex pheromone of M. viciae comprises a synergistic mixture of the nepetalactone I and the nepetalactol II. The identification of aphid sex pheromones will provide a further means of investigating and manipulating the behaviour of these ubiquitous pests.