The taxonomy of gnathiids is solely based on the morphology of the adult males since they are more frequently encountered than the other life stages, therefore female and larval gnathiid descriptions are sometimes absent from species descriptions. Thirty-eight of the almost 180 known gnathiid species are described from Australian waters, with 12 of these collected from the Great Barrier Reef. The only described species parasitising elasmobranchs is Gnathia pantherina Smit and Basson, 2002, a species found on South African sharks and rays. Scientists have examined the relationships between host size and abundance of parasitic larval gnathiid isopods on Heron Island. This is an indication that gnathiids do feed on elasmobranchs but the lack of sampling plays a big role. The objectives of this study are therefore to identify three gnathiid species found on various elasmobranchs from Australia. A phylogenetic analysis will be conducted to confirm whether these species belong to the genus Gnathia Leach, 1814, to determine the relationship between Australian and South African gnathiids species and to see whether there is a correlation between phylogeny and geographic distribution of members of this genus. The statistical analysis using Primer version 5 will be used to provide the specific characteristics responsible for these species groupings seen in the cladograms. Gnathiids were collected from 1996 to 1998 and during March 2002 from various elasmobranchs from Australia. The eight shark and ray species collected from Lizard Island, Heron Island (Shark Bay), and Moreton Bay were; Carcharinus amblyrhynchos Bleeker, 1856, Triaenodon obesus Rüppell, 1837, Rhynchobatus djiddensis Forsskål, 1775, Rhinobatos typus Bennett, 1830, Himantura fai Jordan & Seale, 1906, Taeniura lymma Forsskål, 1775, Urogymnus asperrimus Bloch & Schneider, 1801 and Pastinachus sephen Forsskål, 1775. Larval gnathiid isopods were found attached to the gill filaments and septum of all sharks collected, their numbers and attachment positions were also recorded. After removal from hosts, larvae were kept in fresh sea water until they moulted into adult stages. The males did not conform to descriptions of any known gnathiid species, and were considered to be new to science. The descriptions were based on light and scanning electron microscopy observations. Summary 9 A phylogenetic analysis was conducted to determine where these new species fit into the genus Gnathia and where the new genus Tenerognathia Tanaka, 2005 fits into the family Gnathiidae. The species Caecognathia cryptopais Barnard, 1925 was also included in this study to verify whether this species belongs to the genus Caecognathia Dollfus, 1901. Primer version 5 was used to determine the characteristics responsible for the specific groupings identified by the phylogenetic analysis. The results showed that these three species belong to the genus Gnathia and are only found on Australian elasmobranchs. The findings also contributed to new host records for gnathiids. According to the phylogenetic analysis these new species grouped with the only known elasmobranch gnathiid from southern Africa, G. pantherina. These species made a separate grouping within the Gnathia genus indicating that they have specific characteristics that set them apart from other gnathiids. These characteristics were identified using Primer version 5. It appears that elasmobranch gnathiids are to some extent bigger than teleost gnathiids, have a longer pleotelson and their pylopod’s 2nd article is not circular or conical. However, G. pantherina‘s (from southern Africa) 2nd pylopod article is circular as well as gnathiids from teleosts from the rest of South Africa and Australia. This signifies that this characteristic is host and location specific. The genus Tenerognathia grouped within the genus Gnathia. This might be due to incomplete characteristics information that makes up the data-matrix. The species Caecognathia cryptopais grouped within the genus Caecognathia, confirming that its classification was correct. In conclusion these species are newly described gnathiid species found for the first time on Australian elasmobranchs. This brings the number of known gnathiid species found on elasmobranchs to four. All three life stages were described for Gnathia trimaculata sp. n. and Gnathia australis sp. n and only the male and larvae for Gnathia nigrograndilaris sp. n. The phylogenetic analysis confirmed that there is a correlation between Australian and South African elasmobranch gnathiids, elasmobranchs gnathiids and between phylogeny and geographical distribution. / Dr. N.J. Smit