A taxonomic revision of the genus Galactosomum is proposed within the context of a revision of the subfamily Haplorchinae. The interpretation of the ventrogenital complex underlying the present revision is re-stated in phylogenetic terms in order to call attention to the differentiation of ventral sucker and gonotyl, and of genital sinus, genital sac, and ventrogenital sac. It is inferred from the multiplicity of structure in the terminal genitalia of digenetic trematodes in general, and heterophyids in particular, that they are cross fertilizing despite being hermaphroditic. It is also suggested that variations in terminal genitalia may constitute an important mechanical element in isolating mechanisms that separate closely related species. A general discussion is presented of the taxonomic significance of characters of the following; ventral sucker, gonotyl, lateral pocket, musculature of ventrogenital sac, seminal vesicle, prostatic ejaculatory duct, excretory bladder, vitellaria, body shape, egg size, shape of testes, pharynx, course of uterus, and gland cells. The materials of the revision comprise known specimens, published accounts of these, and new specimens collected in Australia and borrowed from overseas. Wherever possible, types were examined; thus, of 13 species redescribed, types were studied of 11 species, and other specimens of the remaining two. An attempt was made to verify published records by recourse to the specimens on which the records were based; although only partially successful, verification of a number of records has allowed more accurate assessment of host-specificity and geographic distribution of some species. The genus Galactosomum Looss, 1899, is emended, the genera Retevitellus Cable, Connor & Balling, 1960, and Galactosomoides Cable, Connor & Balling, 1960, are reduced to synonymy, and the genera Cercarioides Witenberg, 1929, and Knipowitschiatrema Isaichikov, 1927, are removed from synonymy. The differentiation of Galactosomum from related genera is expressed in the form of a key. The following 19 species, of which five are new, are included. The genotype, G. lacteum Jagerskiold, 1896, is redescribed from syntypes (metacercaria) and compared with adults from the type locality and from Great Britain. G. angelae sp.nov., from Hydroprogne caspia (type host), Sterna bergi, Sula serrator, Eudyptula minor, and Larus novaehollandiae, and as a metacercaria from Hemirhamphus melanochir, all from South Australia, is intermediate between G. bearupi with one group and G. fregatae with two groups of spines on the ventral sucker. G. bearupi sp.nov., from Hydroprogne caspia (type host), Anous minutus, Sterna fuscata, S. bengalensis, and Larus novaehollandiae, and as a metacercaria from Hyporhamphus sp., all from Queensland, differs from G. angelae in having a smaller, rounded ventral sucker armed with a uniform band of spines. G. cochleare (Rudolphi, 1819), orth. emend., is redescribed from three syntypes from Sterna sp. and compared with a specimen from Puerto Rico. G. cochleariformum (Rudolphi, 1819), orth. emend., is redescribed from one syntype from Brazil and five other specimens from the Caribbean. G. darbyi Price, 1934, is redescribed from specimens from Pelecanus occidentalis in Florida and compared with the type series. G. dollfusi sp.nov. (synonym G. cochlear Dollfus & Capron (not Diesing, 1850), 1958) from Sterna hirundo in Senegal, is singular in having a sinistral digitiform process on the gonotyl. G. fregatae Prudhoe, 1949 (synonyms G. agrachanensis Saidov, 1954; G. canis Yamaguti, 1954) is redescribed from syntypes, from specimens referred to G. puffini Yamaguti, 1941, by Caballero, Grocott & Zerecero (1954), Cable, Connor & Balling (1960), Lumsden & Zischke (1963), and Bravo-Hollis (1967) and to Galactosomum sp. by Hutton & Sogandares-Bernal (1960) and from specimens from the domestic dog in Ceylon. G. humbargari Park, 1936 (types untraceable) is redescribed from specimens from a variety of hosts, including the following new hosts: Aechmophorus occidentalis, Podiceps grisegena and Phalacrocorax auritus, from a new locality, British Columbia, Canada. G. johnsoni Price, 1934, is redescribed from the type series, together with other specimens. G. phalacrocoracis Yamaguti, 1939, is redescribed from the type series from Japan, is shown to be distinct from G. lacteum, and is recorded from North America for the first time from Phalacrocorax sp. in Washington State, U.S.A. G. puffini Yamaguti, 1941, is redescribed from the type series, and distinguished from G. fregatae. G. renincola sp.nov., from the renal ureter and bursa Fabricii of Puffinus pacificus (type host), Sterna fuscata, and Anous minutus, in Queensland, is singular in possessing a small, unarmed, asymmetric ventral sucker and a dorsal pit in the forebody. G. sanaensis Kobayasi, 1942, is based on a single specimen since lost; it is included tentatively since, as described and figured, it is distinct from those species seen at first hand. G. sinuilactis sp.nov. from the bursa Fabricii of Phalacrocorax varius (type host) in Queensland (type locality) and South Australia, and of Phalacrocorax fuscescens in South Australia, and as a metacercaria from Haletta semifasciata and Platycephalus sp. in South Australia, is closest to G. renincola, but has a submedian ventrogenital sac with sinistral pseudosucker, a one-chambered seminal vesicle, and a ventral pit in the forebody. G. spinetum (Braun 1901) (synonym Retevitellus spinetus (Braun) Cable, Connor & Balling, 1960) is redescribed from syntypes and compared with specimens from Puerto Rico and Florida; a brief description is given of the metacercaria. G. timondavidi Pearson & Prevot, 1971, is briefly referred to. G. ussuriensis Oshmarin, 1963 (types unobtainable) is redescribed from Larus novaehollandiae, Hydroprogne caspia, and Sterna fuscata in Queensland. G. yehi (Dissanaike) n.comb. (synonym Heterophyopsis yehi Dissanaike, 1961) is redescribed from the type and other specimens from the domestic dog in Ceylon. Species of Galactosomum are assigned to groups according to characters of ventral sucker, seminal vesicle and excretory bladder. Suggested inter-relationships among species and groups are expressed diagrammatically. A key to species is given. A hypothetical description of the mechanics of copulation is attempted on the basis of an interpretation of the functional morphology of the ventrogenital complex, particularly of the complex extrinsic musculature herein described for the first time. It is suggested that opposition of the gonotyls allows mutual insemination to take place. In those species with non-eversible ventral sucker and without a lateral pocket, each gonotyl is supported by the ventral sucker of the partner; in those species with eversible, or protrusible, ventral sucker and with a lateral pocket, the ventral sucker of one partner is received in the lateral pocket of the other partner, and the gonotyls are opposed without direct support from the ventral suckers. In many species, locking of paired flukes is probably aided by one or more extrinsic hemisphincters. In a summary account of the life-cycle, cercarial types are discussed in Galactosomum and related genera, and in opisthorchioids in general. The metacercaria, now known for seven species, attains an advanced stage of development, and can be identified to species on characters of the ventrogenital complex. The hostrange and site in the host of the adult is discussed briefly. All of the species are figured, and all but one of the 96 figures are new.