The extent of allozyme variation and the amount of genetic divergence were assessed among populations of the aphid Rhopalosiphum maidis in Ontario. Allozyme analysis of individuals from 12 sites revealed no polymorphism for 32 loci. In addition, the extent of karyotypic, allozyme, and mitochondrial DNA (mtDNA) variation was examined for clones from North America, Europe, and North Africa. As previously noted, chromosome number varied in relation to the host-plant on which aphids were collected, but neither allozyme nor mtDNA patterns allowed discrimination among clones. Further work is needed to determine whether the lack of genetic diversity in R. maidis is linked with its recent introduction to North America or with its loss of a sexual phase.