The ribosomal DNA cistron from the large raspberry aphid, Amphorophora idaei (Hemiptera: Aphididae), has been mapped by restriction analysis. The results showed that the map of A. idaei was similar to those of the previously characterized aphids Schizaphis graminum and Acyrthosiphon pisum. An extra BgI II site was found in some of the ribosomal DNA intergenic spacer repeats in A. idaei. Using in-situ hybridization to aphid mitotic chromosomes it was demonstrated that probes derived from this region mapped to the pair of X chromosomes and it was therefore aphid in origin. Polymerase chain reaction using conserved rDNA primers also detected significant amounts of a fungal genome in the DNA samples. Microscopic investigation showed that the external surface of A. idaei harboured fungal propagules, hyphae and yeast-like organisms.