The holarctic genus Symmerus and the transantarctic genus Australosymmerus are revised, a reconstruction of their phylogeny is presented, and their zoogeography is discussed. Diagnoses or redescriptions of 31 of the 37 previously-known species and descriptions of 14 new species are presented. The new species are S. nepalensis from Nepal; S. uncatus and S. vockerothi from eastern North America; A. acutus, A. magnificus, and A. truncatus from Mexico; and A. collessi, A. confusus, A. lobatus, A. maculatus, A. magellani, A. minutus, A. montorum, and A. peruensis from South America. Symmerus tristis (Loew) and S. dilutus Fisher are considered synonyms of S. lautus (Loew), and a lectotype is designated for Plesiastina bifasciata Williston. A subgeneric classification is proposed, in which two subgenera of Symmerus and eight subgenera of Australosymmerus are described. Crionisca Colless is considered to be a subgenus of Australosymmerus. The subgenera of Symmerus are Symmerus s. str. and the new subgenus Psilosymmerus (type-species: Symmerus coqulus Garrett). The subgenera of Australosymmerus are Australosymmerus s. str., Crionisca, and the new subgenera Ventrilobus (type-species: Centrocnemis fuscinervis Edwards), Araeostylus (type-species: Australomyia bivittata Freeman), Vellicocauda (type-species: Platyura insolita Walker), Tantrus (type-species: Australosymmerus montorum Munroe), Melosymmerus (type-species: Centrocnemis bisetosa Edwards), and Calosymmerus (type-species: Plesiastina bifasciata Williston). New combinations include A. aculeatus (Edwards), A. bisetosus (Edwards), A. mexicanus (Giglio-Tos), A. nitidus (Tonnoir), A. pediferus (Edwards), A. rieki (Colless), A. simplex (Freeman), A. tillyardi (Tonnoir), A. trivittatus (Edwards), and A. zonatus (Giglio-Tos). In addition to the six previously-described species which were not included in the study, five species remain unplaced in the subgeneric classification. Significant biological data are presented for S. coqulus Garrett, and some observations on the biology of S. vockerothi Munroe are also presented.The phylogeny of Symmerus and Australosymmerus was reconstructed by Hennig's system of "phylogenetic systematics" because there is no clearly-formulated alternative to the cladistic method for phylogenetic analysis. A number of logical and practical difficulties with the procedure are discussed. It was concluded that criteria of deviation from a basic plan and inferred adaptive significance were preferable to criteria of distribution of character states for the recognition and categorization of attributes as plesiomorphic or apomorphic, that the use of minute correspondence in structurally rich characters for the recognition of convergence and synapomorphy is not a reason for discarding phylogenetic methods for phenetic ones, as has been suggested, and that the necessity for tentative reference to a pre-existing phylogeny is neither a philosophical nor a practical problem.For each attribute used in the cladistic analysis, the inferred plesiomorphic and apomorphic state is described and the reason for the inference is stated. The monophyletic group described by each synapomorphy is identified; description of the monophyletic groups at each level specify the phylogenetic reconstruction, which is summarized by means of cladograms.Cladistic and zoogeographic evidence is presented which indicates that transantarctic relationships in Australosymmerus are found in a complex of closely-related sister-groups forming a monophyletic group confined to the South Temperate region. Two of the three transantarctic relationships occur at the intra-subgeneric level; the remaining one occurs at the inter-subgeneric level. The remaining species of Australosymmerus are shown to form a monophyletic morphological-chorological progression from south to north across the equator in the New World. The evidence indicates that the ancestor of Symmerus and Australosymmerus lived in the northern hemisphere. One phyletic line dispersed to the southern hemisphere and reached what is now South America at least by the early Tertiary, but probably much earlier, and gave rise to Australosymmerus. If Australosymmerus did not cross large water gaps, it must have been present in Gondwanaland before the break-off of New Zealand (Lower Cretaceous). Australosymmerus is not present in South Africa, indicating that it did not exist in Gondwanaland before the break-off of South Africa, or that it was present and has subsequently become extinct in South Africa. The sister-group of the transantarctic group migrated northwards, probably reaching northern South America before the end of the Oligocene.