Multilocus enzyme electrophoresis was used to compare specimens of the parasitic nematode Cloacina obtusa from the stomach of the eastern grey kangaroo, Macropus giganteus and the western grey kangaroo, M. fuliginosus. Allelic variation among nematodes was detected at 17 (85%) of 20 loci, but there was only a single fixed genetic difference (at the locus for isocitrate dehydrogenase, IDH) between C. obtusa from M. fuliginosus and those from M. giganteus in areas where each host occurred in allopatry. However, this fixed difference was not apparent within the zone of host sympatry. Although electrophoretic data indicate genetic divergence among allopatric populations of C. obtusa in the two host species, the magnitude of the electrophoretic difference (5%) between these populations does not refute the hypothesis that C. obtusa represents a single species. The ’usual’ situation for parasitic helminths of grey kangaroos is that pairs of parasite species occur in the two host species. This situation differs for C. obtusa, where there has been a lack of speciation following a speciation event in its macropodid marsupial hosts. This finding suggests that a speciation event in the host does not necessarily lead to a speciation event for all its parasites and further highlights our lack of understanding of which processes drive speciation in parasites.