An electrophoretic study of Pelecitus roemeri from Macropus robustus, M. giganteus and Wallabia bicolor revealed no genetic differences at 23 enzyme loci. The genetic data support the existing morphological evidence that P. roemeri from these three hosts represents a single species. The data show no genetic variation between nematodes from the same or different host species collected in northern and southern Australia. This result is discussed briefly in relation to Price's model of parasite speciation.