The phylogeny of semi-aquatic bugs (Hemiptera-Heteroptera: Gerromorpha) was tested in parsimony analyses of 64 morphological characters and approximately 2.5 kb of DNA sequence data from the mitochondrial genes encoding COI+II and 16SrRNA and the nuclear gene encoding 28SrRNA. The taxon sample included representatives of all families and most subfamilies of Gerromorpha and a selection of outgroup taxa representing the two basal infraorders of Heteroptera, Enicocephalomorpha and Dipsocoromorpha, and two families of Nepomorpha. A simultaneous analysis (SA) of all data, and with gaps scored as fifth state characters, gave a single most parsimonious tree with all families resolved as monophyletic, except the Veliidae, where Microveliinae + Haloveliinae, Veliinae, Rhagoveliinae, Perittopinae, and Ocelloveliinae were resolved as successive sister groups to the Gerridae, thus confirming earlier statements about paraphyly of this family. The Gerridae + Veliidae clade was strongly supported, but otherwise only the Gerridae + Veliidae less Ocelloveliinae and the Gerridae itself had support. These three clades could all be diagnosed on apomorphic morphological characters, although no characters diagnosing the Gerridae were without convergences or present in all included taxa. While the Ocelloveliinae, Veliinae and Haloveliinae could not be diagnosed on convincing apomorphies, the Microveliinae + Haloveliinae, and their sister group relationship with the Gerridae, could be diagnosed on rather strong morphological synapomorphies, suggesting that Gerridae could be expanded to include these two veliid subfamilies, while Ocelloveliinae, and perhaps the remaining veliid subfamilies, could be elevated to new families. In Gerridae, the Ptilomerinae + Halobatinae was sister group to all other subfamilies, while the Rhagadotarsinae + Trepobatinae was sister group to a clade comprising the Gerrinae, Eotrechinae, Cylindrostethinae and Charmatometrinae. Most relationships in this clade were poorly supported and diagnosed, and Cylindrostethinae was surprisingly found to be paraphyletic. The sister group to the Gerridae + Veliidae clade was a strongly supported clade comprising the Paraphrynoveliidae and Macroveliidae, and this, and the lack of convincing synapomorphies for Paraphrynoveliidae, suggest that these two small families could be synonymized. For the basal relationships of Gerromorpha, the Mesoveliidae was strongly supported sister group to all other families, while the Hebridae, Hermatobatidae and Hydrometridae formed a poorly supported and poorly diagnosed sister group to the Gerridae + Veliidae + Paraphrynoveliidae + Macroveliidae clade. The unexpected sister group relationship between Hermatobatidae and Hydrometridae was moderately supported, and could be diagnosed on two synapomorphies, thus giving a new hypothesis about the relationships of these very divergent families. Phylogenetic analyses of individual character partitions gave less resolved and less supported relationships, and the mitochondrial genes COI+II and 16SrRNA contributed negative hidden partitioned Bremer support (HPBS) to the simultaneous analysis tree, probably due to homoplasy caused by saturation effects.
