The larva ofHeterocloeon frivolus(McDunnough) comb. n. is described for the first time. Larval morphology indicates that the species be removed fromBaetisand placed in the monophyletic genusHeterocloeonand indicates a relatively ancestral position of the species among congeners ofHeterocloeon.On the basis of considerable new data, a previous phylogeny is falsified in part and a new phylogeny ofHeterocloeonis hypothesized.Heterocloeon petersiandberneriare considered to be advanced sibling species. It is also deduced thatHeterocloeonshareda recent commonancestor withPseudocloeonand certain derivedBaetis.