In parthenogenetic females of a clone of the aphid Megoura viciae (Homoptera, Aphididae), more than 50% of the cells show heteromorphism between homologous NORs which are located on one telomeric region of the two X chromosomes. Using different techniques, such as staining with the CG-specific fluorochrome chromomycin A(3), silver staining and in-situ hybridization with an rDNA probe, we have shown that the observed heteromorphism is due to an unequal distribution of ribosomal genes between homologous NOR regions. The total number of rDNA genes per individual aphid remained constant. Moreover, the analysis of cells from single embroys has shown that the observed heteromorphism is not only intraclonal but also intraindividual. These data, together with the finding of X chromosomes connected by chromatin bridges between their NORs, allow us to suggest that mitotic unequal crossing over could be the main cause of NOR heteomorphism in this taxon.