Abstract The cornicle secretions from 28 species of aphids were examined directly by mass spectrometry and, in selected cases, by chemical and chromatographic methods. The major components of the secretions are triglycerides, the acid moieties of which are hexanoic, hexadienoic (sorbic), tetradecanoic (myristic), and hexadecanoic (palmitic) acids, with smaller amounts of C 12 and C 18:1 acids. The mixtures from the different species fall into six groups which are readily distinguished by mass spectrometry. The composition was independent of the host plant or of the stage of development where these have been investigated. Biochemical classification is not consistent with morphological classification in the species examined.