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Background: The strongyloid nematode subfamily Phascolostrongylinae are parasites of the large intestine and stomach of Australian macropods (Macropodidae) and wombats (Vombatidae). Based on morphological classifications, the Phascolostrongylinae is comprised of seven genera belonging to three tribes (Phascolostrongylinea, Macropostrongyloidinea, and Hypodontinea). The phylogenetic relationships among the genera of the Phascolostrongylinae were tested using the first and second internal transcribed spacer (ITS+) sequences of the ribosomal DNA. Results: Monophyly was encountered in the tribe Phascolostrongylinea comprising two genera, Phascolostrongylus and Oesophagostomoides, found exclusively in the large intestine of wombats. The tribe Hypodontinea, represented by the genera Hypodontus and Macropicola from the ileum and large intestine of macropods was also found to be monophyletic, but with low support. The tribe Macropostrongyloidinea comprising the genera Macropostrongyloides and Paramacropostrongylus was paraphyletic with the species occurring in the stomach grouping separately to those found in the large intestines of their hosts. Finally, Macropostrongyloides dissimilis from the stomach of the swamp wallaby and Paramacropostrongylus toraliformis from the large intestine of the eastern grey kangaroo were distinct from their respective congeners. Conclusion: The current study provided strong support for the generic composition of the tribe Phascolostrongylinea and low support for the tribe Hypodontinea. However, the relationships within the tribe Macropostrongyloidinea are more complex and its monophyly was not supported by the current ITS+ dataset. The unexpected finding of M. dissimilis and P. toraliformis being distantly related to their respective congeners suggests a requirement for future taxonomic revision which may warrant separation of these species at the generic level.

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