Nine new species of didymozoid (Platyhelminthes, Digenea) are described from five species of platycephalid (flathead) fish collected in Moreton Bay, Australia. They are: lndodidymozoon moretonensis sp. n. from the buccal cavity and branchial arch'es, I suttiei sp. n. from the skeletal muscles, and Rhopalotrema elusiva gen. n., sp. n. from the fins of Platycephalus fuscus; I lesteri sp. n. from the belly skin, I pearsoni sp. n. from the eyes, and Helicodidymozoon tortor gen. n., sp. n. from the branchial arches, operculum and buccal cavity of P. endrachtensis; R. pterygionastes sp. n. from the fins of P. indicus; I metridion sp. n. from the operculum of Suggrundus jugosus; and I ditremion sp. n. from_ the buccal cavity, branchial arches and oesophagus of lnegocia japonica. The taxonomic composition of the genus Didymozoon is discussed. Five species, D. branchiale, D. brevicolle, D. koti, D. musculi and D. platycephali, are moved to the genus Jndodidymozoon; the generic diagnosis of lndodidymozoon is revised. lndodidymozoon brevicolle from the stomach wall of P. endrachtensis and P. fuscus is redescribed. One species in the genus Didymozoon, D. spirale, and a species from the genus Neometadidymozoon, N helicis, are moved to the new genus Helicodidymozoon. Helicodidymozoon differs from Didymozoon and lndodidymozoon in the structure of the hindbody which is spirally twisted rather than straight and in having a forebody-hindbody junction that is subterminal rather than ventral. The genus Rhopalotrema is erected to accommodate the two new species R. elusiva and R. pterygionastes. The sequences of the second internal transcribed spacer (ITS2) of the ribosomal DNA (rDNA) of 11 species of didymozoid, I brevicolle, I ditremion, I. lesteri, I. metridion, I. moretonensis, I. pearsoni, I. suttiei, H helicis, H tortor, R. elusiva and Neometadidymozoon polymorphis and the cercaria of a hemiuroid species (Lecithqster sp.?) are used to confirm species boundaries and to infer a phylogeny. The phylogenetic analyses indicate that the species in the genera Helicodidymozoon and Rhopalotrema represent a monophy letic group, supporting separation of the species in the genus Helicodidymozoon from the genera lndodidymozoon and Neometadidymozoon and the recognition of Rhopalotrema as a genus distinct from Neometadidymozoon. It is suggested that the levels of variation among the nucleotide sequences of the species indicate that the gonochoristic species, I. pearsoni and I suttiei, should not be placed in a different subfamily or family to the other species of didymozoid examined. The pattern of diversity of didymozoids amongst the flatheads of Moreton Bay is discussed. Individual host species harbour between zero and six species of didymozoid. Host species from the Platycephalus-stem harbour species from up to three genera, lndodidymozoon, Helicodidymozoon and Rhopalotrema, whereas hosts from the Onigocia-stem harbour species only from the genus lndodidymozoon. All of the didymozoid species except one, lndodidymozoon brevicolle, are strictly host-specific. This strict specificity is attributed to interactions between the host and parasite rather than environmental influences. Species of didymozoid occupy clearly defined sites within their host with no overlap between congeneric species but some overlap between species from different genera. The relatively high diversity of didymozoids in flathead fishes is attributed to the patterns of host- and site-specificity and to the colonisation of flatheads by three independent lineages of didymozoids. I conclude that flatheads that lack didymozoid parasites probably do so because of habitat and diet. The sequence of the ITS2 of one type of didymozoid metacercaria identifies it as the metacercaria of I pearsoni. The metacercaria of I pearsoni and the changes that occur as it develops into the adult stage are described. Taxonomic features that may be useful for matching metacercariae to adults are discussed. The 'stomach' present in some metacercariae probably represents the chamber at the posterior end of the oesophagus of some adults and its presence or absence may be of some use in the preliminary identification of metacercariae. The sizes of the oral and ventral suckers of I pearsoni remain the same as the fluke grows from metacercaria to adult. If this is true for other species of didymozoid, the size of the suckers will be valuable for making a preliminary identification of didymozoid metacercariae. Sequencing of the ITS2 is an effective method for identifying didymozoid metacercariae. Two types of cystophorous cercaria are described. Both were collected from small intertidal molluscs. The sequences of the ITS2 of the two species differ considerably from those of the didymozoids studied. I suggest, because of morphological similarities between the two cercariae and previously described cercariae of species in the genus Lecithaster, that the new cercariae are from the family Lecithasteridae and probably the genus Lecithaster. I propose that the position of the oral sucker of cystophorous cercariae indicates whether the cercaria is from the Didymozoidae or another hemiuroid family because the oral sucker of the metacercariae and adults of didymozoids is almost always terminal whereas the oral sucker of cercariae, metacercariae and adults of other hemiuroid families is usually subterminal.