Revised diagnoses of the subfamily Prostatomicrocotylinae and the largest of its five genera, Polylabris, are presented. Seventeen species of Polylabris are recognised from the gills of a variety of perciform fishes on the basis of the morphology of their male copulatory organs and a key to species is given; a further three species inquirendae are also considered. P. sillaginae (Woolcock) is recorded in Australia, New Caledonia and the Gulf of Thailand from ten species of Sillago and the type host Sillaginodes punctata. P. sandarsae Williams is synonymised with P. sillaginae (Woolcock), as are P. sp. 1 Williams and P. sp. 2 Williams. Three new bivaginate species from Australian sillaginids, each with distinctive copulatory organs, are described: P. australiensis, sp. nov., P. queenslandensis, sp. nov., and P. williamsi, sp. nov. The only species in this host group with one vagina, P. madagascarensis, sp, nov., is described from Sillago sihama in Madagascar and South Africa. Two other new univaginate species, from eastern Australia, are described: P. girellae, sp. nov., from a kyphosid (Girella tricuspidata) and P. rhabdosargi, sp. nov., from a sparid (Rhabdosargus sarba). P. gerres (Machida) is recognised as both a junior synonym and secondary homonym of P. gerres (Sandars). P. mamaevi Ogawa & Egusa is likely to be composed of two species, one of which may be synonymous with P. virgatarum (Tubangui), a species whose types were destroyed. Gupta & Khullar's 'P. sillaginae' is given a replacement name, P. indica, nom. nov., because it undoubtedly belongs to a distinct species. In four species with no available specimens, original illustrations of copulatory organs are redrawn. New species are still very likely to be discovered in the Indo-west Pacific, particularly in regions where the monogenean fauna is little known, such as eastern Africa and the Red Sea.
