We have used a new technique to identify discrete genetic markers in aphids, a family in which biochemical and morphological genetic polymorphisms are rare. The new technique uses the polymerase chain reaction (PCR) to amplify random regions of aphid genomes (random amplified polymorphic DNA) and has been termed RAPD-PCR. We demonstrate the use of the technique in revealing genetic variation in four aphid species, the greenbug (Schizaphis graminum (Rondani)), the Russian wheat aphid (Diuraphis noxia (Mordvilko)), the pea aphid (Acyrthosiphon pisum (Harris)), and the brown ambrosia aphid (Uroleucon ambrosiae (Thomas)). In contrast with allozyme surveys, RAPD-PCR revealed large amounts of genetic variation among individuals in each of these species. Variation was detected among biotypes, populations, colour morphs and even individuals on a single plant. We also explored the utility of RAPD-PCR in the detection and identification within aphid bodies of two endoparasitic wasps, Diaeretiella rapae (McIntosh) and Lysiphlebus testaceipes (Cresson). The use of RAPD-PCR in species diagnostics, parasitoid detection, and population studies is discussed.