The mass spectra in both instances were nearly identical to that reported for trans-~-farnesene•. Synthesis and mass spectroscopy of trans-~-farnesene revealed that this compound and the aphid odour were identical. Bioassay of trans-~farnesene was carried out by crushing a sealed glass ampoule containing approximately 10 mg of synthetic trans-~-farnesene in the presence of M. persicae on turnip leaves. A large number responded by falling or walking from the plant.Combined with insecticidal sprays for the control of aphids, the alarm pheromone could enhance their effectiveness. The pheromone will cause those aphids avoiding a direct hit by a spray droplet to walk over the plant and contact the insecticide.This work was supported by a Federal Hatch Act grant. We thank Dr L. R. Nault for facilities and discussion of unpublished research and D. Henderson for technical assistance.Note added in proof At the time this manuscript was submitted for publication Dr W. S. Bowers, USDA, Beltsville, Maryland, informed us that he had established that trans-13farnesene was also the alarm pheromone for the rose aphid, Macrosiphum rosae.