AbstractA prerequisite to studying the specific interactions involved in the persistent transmission of luteoviruses such as the potato leafroll virus (PLRV) is the characterization of both the virus and its vectors. A range of techniques was used to assess genetic differentiation among 27 clones belonging to theMyzus persicaecomplex (M. persicae(Sulzer),M. antirrhinii(Macchiati) andM. nicotianaeBlackman) and showing different efficiencies in transmitting PLRV isolates. AllM. persicae/M. nicotianaeclones belonged to one of two karyotypes, both 2n = 12, either normal or carrying an autosomal translocation (A1,3), and allM. antirrhiniiclones had 13 or 14 chromosomes. Amplified esterase 4 genes were detected by PCR–REN assay inM. persicae/M. nicotianaetaxa, with gene expression being modified by methylation. Similarly, amplified E4 genes were revealed inM. antirrhiniibut they all showed unmethylated. Two allozyme and 11 microsatellite loci discriminated 10 different genotypic classes among the 27 clones. Analysis of genetic relatedness between these genotypic classes revealed thatM. nicotianaeclones were very closely related toM. persicaeclones, whereas the genetic differentiation betweenM. antirrhiniiandM. persicaewas greater. The implications of these results for the taxonomic status of these genotypes within the complex, and the transmission of PLRV, are discussed.