The taxonomy and biology of the Polystomatidae of Australian hosts was studied. One species of polystomatid was found in the lungfish, nine species in freshwater turtles, and two species in frogs. The validity of characters used in taxonomy of the Polystomatidae is reviewed and assessed. Characters useful to separate species include: the number of genital spines; the shape of hamuli; the shape of the testis; the shape of eggs; and the outline of the gut. Pseudopolystoma australensis is redescribed from new material collected from natural populations of the lungfish, Neoceratodus forsteri, in Australia; the polystomatid is transferred to a new genus, Concinnocotyla, as Concinnocotyla australensis n. comb., in a new subfamily, Concinnocotylinae. The new genus possesses an elaborate skeleton of sclerites in each haptoral sucker, a pocket on each caecum that opens dorsally, a sperm-filled sac between the pockets, numerous testes, and a muscular penis which lacks genital spines. Eggs of C. australensis are attached, in large numbers, by appendages to the tooth plates of lungfish. Body-washings and mucus of the host did not stimulate eggs of C. australensis to hatch; eggs hatched spontaneously. Only unciliated oncomiracidia were recovered from freshly hatched eggs. Results of experimental infections suggest that adult polystomatids rather than oncomiracidia transfer from host to host and thereby establish new infections. Parapolystoma johnstoni n. sp. is described from Litoria nyakalensis from northern Queensland. Parapolystoma bulliense is recorded from a new host, Litoria pearsoniana. Parapolystoma is removed from the Polystomatinae and placed in the Diplorchiinae because it has a long uterus that extends past the ovary into the haptor, it has more than one testis and its oncomiracidium has 59 ciliated cells. Parapolystoma croold is probably not congeneric with P. bulliense because it has fewer ciliated cells, a differently shaped uterus, and a different type of testis. Polystomoides australiensis was recovered from the urinary and accessory bladders of Emydura signata, Emydura macquarii and Elseya latisternum and is redescribed from unflattened specimens. Polystomoides scottae n. sp. is described from specimens collected from the urinary and accessory bladders of Chelodina expansa. Four new species are described from the conjunctival sacs of Australian freshwater turtles (Chelidae): Neopolystoma cribbi n. sp. from Emydura signata, Emydura macquarii, Elseya latisternum and Chelodina expansa; Neopolystoma spratti n. sp. from Chelodina longicollis; Neopolystoma tinsleyi n. sp. from Chelodina expansa; and Neopolystoma queenslandensis n. sp. from Emydura signata and E. macquarii. Neopolystoma macleayi is redescribed from new unflattened specimens; new host records and geographical locations are given. Neopolystoma australis is made a junior synonym of Neopolystoma kreffti and new host and geographical locations are given for N. kreffti. Neopolystoma chelodinae is redescribed from a new geographic location and possibly from a new host. Polystomoides australiensis, Neopolystoma macleayi and N. cribbi produced on average 5.6, 4.3 and 0.7 eggs per day respectively. Host urine did not stimulate eggs of P. australiensis and N. macleayi to hatch. Eggs of P. australiensis take 33-57 (40.9) d (n=14) at 18-24°C and 30-33 (31.4) d (n=7) at 24-28°C to develop and hatch. All oncomiracidia of P. australiensis and N. macleayi had 64 ciliated cells. Successful cross-infection experiments between host genera (Emydura to Elseya) indicated that P. australiensis and N. macleayi have a low degree of host specificity. Host choice, therefore, cannot be used as a taxonomic character when differentiating between species of the subfamily Polystomoidinae. N. macleayi and P. australiensis migrate to the ureters and possibly to the kidneys. The different sizes of worms in different sites in the hosts suggests strongly that P. australiensis migrate from the ureters, to the urinary bladder and finally to either the accessory bladders or cloaca. The taxonomy of North American polystomatids is confused; many of the type species deposited in museums were poorly prepared, incorrectly labelled, and are often represented by only a single specimen. The problem cannot be rectified without new material. The key to understanding trends in the evolution of the Polystomatidae lies in the morphology of the haptoral suckers, the number of ciliated cells on the oncomiracidia, their geographical distribution, the hosts they parasitize and the degree of host specificity. From this information it is concluded: that Concinnocotyla is the sister group to the remainder of the Polystomatidae including the Sphyranuridae and that its lineage separated possibly as early as the Devonian; that many of the polystomatines can be derived from a Polystoma-like form with one or two modifications; that diplorchiines probably separated from polystomatines early; and that polystomoidines and Oculotrema have a lineage in common with Protopolystoma.