Mi is a dominant locus in tomato that confers resistance to both root knot nematodes, Meloidogyne spp., and potato aphids, Macrosiphum euphorbiae (Thomas). The goal of this study was to investigate the breadth of Mi-mediated aphid resistance. To examine variability in potato aphid response, the effect of Mi on seven isolates of the potato aphid from California, New Jersey, North Carolina, France, and The Netherlands was measured in greenhouse bioassays. Assays were also performed to determine if Mi impacted another aphid species that attacks tomato, Myzus persicae (Sulzer). Tomato varieties carrying Mi had a profound negative impact on the potato aphid isolates from France and The Netherlands. In contrast, other potato aphid isolates from New Jersey, North Carolina, and California proved to be virulent, or capable of overcoming Mi. Mi-mediated resistance also did not extend to the green peach aphid isolates tested. These results demonstrate that Mi-mediated aphid resistance is isolate-specific, and it may not extend to all aphid species. Our data also show that virulent potato aphids are geographically widespread, and, unlike virulent nematode isolates, are relatively common. Virulence is persistent and is maintained even in the absence of selection pressure by Mi. The response of a particular aphid isolate to Mi could not be predicted on the basis of aphid color, because both avirulent and virulent isolates were found within the pink and green biotypes of the potato aphid.