The genetic diversity and phylogeny of western Palaearctic members of the Gerris lacustris group was investigated on the basis of 822 bp from the 3´end of the mitochondrial gene encoding COI obtained from 34 specimens of G. lacustris, 16 specimens of G. gibbifer, eight specimens of G. maculatus and seven specimens of G. brasili. Nine haplotypes represented G. lacustris, nine haplotypes represented G. gibbifer, six haplotypes represented G. maculatus, four haplotypes represented G. brasili, and a single haplotype was shared between G. gibbifer and G. brasili. Uncorrected p genetic distances within species averaged from 0.5% in G. gibbifer and G. brasili to 0.8 in G. lacustris and as much as 2.2 in G. maculatus. A phylogenetic analysis showed that G. gibbifer and G. brasili are not reciprocally monophyletic in their mtDNA probably due to relatively recent speciation and incomplete lineage sorting. G. maculatus was poorly supported as the sister taxon to G. gibbifer and G. brasili despite morphological similarities, and a clear division between G. maculatus populations in the eastern and western Mediterranea, may indicate different refugia during the Pleistocene glaciations. G. lacustris showed little genetic substructure except for some isolated haplotypes in Italy, Turkey and Armenia, but these are geographically overlapping with more widespread haplotypes, and probably represent more ancient diversities in these regions.