The diversity of New Zealand and Australian asellote isopods remains largely unknown. Even less is known about how this fauna is related to the rest of the world. This study focuses on the munnopsid subfamily Ilyarachninae Hansen, 1916, to explore the phylogenetic and biogeographic relationships of the local fauna to others from this group worldwide. To analyse the relationships within the Ilyarachninae, a suitable outgroup, in this case, the sister group, was needed to be determined. To do this, a cladistic analysis of the Munnopsidae was undertaken. In addition to determining a suitable outgroup, the family level analysis also served to test the monphyly of the nine subfamilies and to resolve the placement of the three genera, Munneurycope Stephensen, 1913, Munnicope Menzies and George, 1972 and Munnopsurus Richardson, 1912b which were not placed in any exsiting subfamily. The cladistic analysis of the Munnopsidae included 72 species representing all munnopsid genera, including where possible, the type species and was based on 122 morphological characters. A well resolved strict consensus tree was found. The results show Munnopsurus to be the sister group of the Ilyarachninae and the Munnopsinae to be the most ancestral group of the Munnopsidae. Of all the subfamilies, the Betamorphinae is not a monophyletic. The results show that although some subfamily names are upheld, others show no congruence to the topology of the results, and therefore the use of subfamily names are abandoned. Four families are instead recognized, the Munnopsidae (restricted to the former Munnopsinae), Acanthocopidae Wolff, 1962, the Syneurycopidae Wolff, 1962 and the Lipomeridae Tattersall, 1905a, with the latter including the former Ilyarachninae ('Ilyarachna-group'). Within the Ilyarachna-group, two new genera and 28 species were described, including 25 species new to science. Two genera, Notopais and Epikopais n. gen. are found exclusively in the southern hemisphere. Of the more cosmopolitan genera, some New Zealand and south-east Australian species display close affinities to each other and with Antarctic species. These relationships were tested using cladistic phylogenetic methods, and in addition to this, the analysis also served to test the monophyly of the genera. The analysis used 59 species, representing all genera and 61 morphological characters. The strict consensus tree was well resolved, however Aspidarachna Sars, 1897b, Ilyarachna Sars, 1870 and Echinozone Sars, 1897b were not monophyletic. The results also showed that the Ilyarachna-group fauna of New Zealand and south-east Australia are closely related to each other and to species from Antarctica.