Aphis is the largest aphid genus in the world and contains several of the most injurious aphid pests. It is also the most reluctant aphid genus to any comprehensive taxonomic treatment: while most species are easily classified into "species groups" that form well defined entities, numerous species within these groups are difficult to tell apart morphologically and identification keys remain ambiguous and mostly rely on host plant affiliation. In this paper, we used partial sequences of COI/COII and CytB genes to reconstruct the first phylogeny of Aphis and discuss the present systematics. The monophyly of the subgenus Bursaphis and of the tree major species groups, Black aphid, Black backed aphid and frangulae-like species was recovered by all phylogenetic analyses. However our data suggested that the nominal subgenus was not monophyletic. Relationships between major species groups were often ambiguous but "Black" and "Black backed" species groups appeared as sister clades. The most striking result of this study was that our molecular data met the same limits as the morphological characters used in classifications: mitochondrial DNA did not allow the differentiation of species that are difficult to identify. Further, interspecies relationships within groups of species for which taxonomic treatment is difficult stayed unresolved. This suggests that species delineation in the genus Aphis is often ambiguous and that diversification might have been a rapid process.