New material from Epinephelus quoyanus collected from Heron Island, Australia and material collected by Young in the same locality, allegedly from E. merra, and deposited in the Queensland Museum, contained the same three species of diplectanid monogeneans: Pseudorhabdosynochus cupatus (Young, 1969), P. vagampullum (Young, 1969), and P. justinei Zeng et Yang, 2007 (new record for Australia). In contrast, E. merra in New Caledonia harbours only P. melanesiensis (Laird, 1958). It is concluded that the type-host of P. cupatus and P. vagampullum is not E. merra, as indicated in Young (1969), but E. quoyanus, and that P. cupatus sensu Young comprises two species, P. cupatus from E. quoyanus and P. youngi sp. nov. from E. fasciatus. P. youngi sp. nov. is described from E. fasciatus from New Caledonia (type-locality), Heron Island, and French Polynesia, and distinguished from P. cupatus on the basis of the male quadriloculate organ, sclerotised vagina and lamellosquamodiscs. Thus, P. cupatus, P. melanesiensis and P. youngi each have a single host, respectively E. quoyanus, E. merra, and E. fasciatus. Results for E. fasciatus and E. merra in New Caledonia, Wallis and French Polynesia suggest impoverishment of the monogenean fauna in a West-East gradient in the Pacific.