This study describes the helminth community in 28 Rattus leucopus, from Australia, Papua New Guinea and Papua (formerly Irian Jaya), and compare the community in Australian R. leucopus with those known from two other Australian endemics, R. sordidus and R. fuscipes. In total, 28 parasite species were found: one acanthocephalan, five cestodes, one trematode and 21 namatodes as well as two lots of unidentified spirurid nematode lavae. The diversity of helminths in R. leucopus was high but equitable; the reciprocal of Simpson's Index was 1.75. The distribution of helminth species was not indicative of a core-satelite structure. No species occurred at a prevalence >30% and more than half the species occurred at a prevalence of <10%. Only 10% of the helminth species identified were unique to R. leucopus. Helminth community structures in R. leucopus differed from that observed in R. sordidus and the community in R. fuscipes was markedly different from those in R. leucopus and R. sordidus. Determinants of the diversity of the helminth community in R. leucopus include the time intervals between the three waves of migration of Rattus spp. to Australia, biogeographic factors related to immigration and adaptive radiation of the host genus, and associated with coevolution, speciation and host switching of the helminths.