The taxonomy of the Gyrodactylidae (Monogenea) from marine and freshwater fishes of the Australian continent is examined for the first time. New species are described for three known genera (Anacanthocotyle, Gyrodactylus and Polyclithrum) and a new, viviparous genus is also described. The new genus of viviparous gyrodactylid is described from the fins and skin of siganid fishes from the Great Barrier Reef (GBR), Queensland, Australia. The genus is characterised by a muscular, tube-like haptor with 16 marginal hooks on the posterior margin. The ventral lobe of the haptor is located anteriorly relative to the dorsal lobe and contains a pair of hamuli and a ventral bar with a posteriorly-projecting ventral bar membrane. A dorsal bar is absent. Species of this genus differ from other gyrodactylids by having a bilateral excretory system consisting of only six pairs of flame bulbs. Seven new species are described from Siganus corallinus, S. fuscescens, S. lineatus, S. puellus, S. punctatus, and S', vulpinus. Species of the new genus were observed to attach using only the marginal hooks and the role of hamuli in attachment is unclear. The dorsal fin of the host is the preferred site for most species but the anal fin, caudal fin and body surfaces are preferred by some species. A new species of Anacanthocotyle is described from Stigmatopora nigra (Syngnathidae) from Moreton Bay, southeast Queensland. The bilateral excretory system of this species has eight paired flame bulbs and is similar to species in the Gyrodactylus (Metanephrotus) subgenus. This species differs from the single describedspecies of Anacanthocotyle and other members of the Isancistrinae by having lateral gut diverticula and gut caeca that terminate blindly. Ten species of Gyrodactylus are reported from marine and freshwater fishes. Three of these Gyrodactylus species are from native Australian freshwater fishes. Gyrodactylus anguillae was previously known only from anguillid eels native to freshwater catchments of the North Atlantic Ocean but is here reported from Anguilla australis and Anguilla reinhardtii in Australia. Two species of Gyrodactylus are described from the fins and skin and the gills of Galaxias olidus. Three introduced species of Gyrodactylus are redescribed from wild populations of exotic fishes in Australia; namely, Gyrodactylus bullatarudis from Poecilia reticulata and Xiphophorous helleri, G. kobayashii from Carassius auratus, and G. macracanthus from Misgurnus anguillicaudatus. An unidentified species of Gyrodactylus was discovered on Tanichthys albonubes held in quarantine after importation into Australia. The potential for further introduction of Gyrodactylus species into Australia and the potential consequences are discussed. Three marine species of Gyrodactylus are described. These species are reported from Apogon compressus and A. cookii from the northern GBR from Istiblennius meleagris from the southern GBR and from Pelâtes sexlineatus and Sillago maculata from Moreton Bay, southeast Queensland. Polyclithrum mugilini Rogers, 1967, a parasite oï Mugil cephalus, is redescribed from type material from Lake Seminole, Georgia, USA. Three new species of Polyclithrum Rogers, 1967 are also described from M. cephalus from the Albert River, Queensland, Australia, from M. cephalus from Heron Island, GBR, Australia and from M. platanus from Rio da Guarda, State of Rio de Janeiro, Brazil. The validity of Micropolyclithrum parvum Skinner, 1975, a parasite of Mugil cephalus in Biscayne Bay, Florida is discussed, and a key to the species of Polyclithrum is presented. The structure and possible functions of posterior cells and syncytia (=type-l ’cells'), described previously in gyrodactylids as vitellaria, are examined. In G. macracanthus they are individual cells with a single nucleus whereas in G. kobayashii they form multinucleate syncytia. The chemical nature and ultrastructure of these cells differs from the vitellarium of oviparous monogeneans in that they contain no glycogen, lipid or phenolic substances but are rich in rough endoplasmic reticulum and stain intensely for proteins. Possible relationships of type-1 'cells' with adjacent cells and organs are discussed. A hypothetical evolution of these cells is proposed assuming their homology with the vitellarium of oviparous monogeneans. The birth pore and process of birth in Gyrodactylus kobayashii is examined. The birth pore consists of a basal matrix that joins to those of the uterine lining and tegument. The basal matrix is overlain by a vesicular cytoplasm that contains few organelles. There are no muscles directly associated,with the birth pore. The birth pore is a preformed structure that is present in new bom worms. The birth pore does not appear to act as a tube or canal extending from the uterus to the exterior. The small size of the structure in comparison with the large embryo means that it can be described as a dilatable area in the body wall adjacent to the uterus. The elastic nature of tissues surrounding the birth pore appears to be óf vital importance. Their elasticity is discussed in relation to the composition of their extra cellular matrices.