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Nematodes of the superfamily Strongyloidea that inhabit the gastrointestinal tracts of Australasian macropodid and vombatid marsupials are one of the most speciose groups of mammal parasites. These nematodes are prevalent and occur in high abundance in almost all species of macropodid and vombatid marsupials. A review of the literature (Chapter 1) revealed that most studies of strongyloid nematodes of marsupials are species descriptions and taxonomic revisions, based on morphological features. Currently, there are over 300 morphospecies, most of which occur in the stomachs of macropods and are placed in the subfamily Cloacininae, and the remaining species, found mostly in the intestines of macropods and wombats, represent the subfamily Phascolostrongylinae. From the early 1990s onwards, the use of molecular approaches in taxonomic studies accelerated the discovery of marked population variation and cryptic species, allowed relationships of taxa to be resolved and provided improved insight into speciation processes. However, studies combining molecular and morphological approaches have been conducted mainly on the subfamily Cloacininae, whereas the subfamily Phascolostrongylinae has been neglected, resulting in very limited knowledge of the systematic status of this subfamily. Additionally, a monophyletic hypothesis has been proposed for the subfamilies Cloacininae and Phascolostrongylinae; however, the relationship between these subfamilies based on morphological features has not been established beyond speculation. To address these gaps, the research aims of this thesis were to assess the genetic variability within the Phascolostrongylinae, elucidate any species complexes and determine their phylogenetic relationships using sequence data sets derived from the internal transcribed spacers (ITS) of nuclear ribosomal DNA and/or the inferred mitochondrial proteome. This thesis also aimed to determine the relationship between the Phascolostrongylinae and Cloacininae to