Fifteen helminth species—the acanthocephalan Moniliformis moniliformis, the cestodes Hymenolepis cf. diminuta and Paroniella sp., and 11 nematode species identified at least to subfamily and 1 to family—were found in the digestive tracts of 10 individuals of Rattus mordax (Muridae) from Papua New Guinea. Of the nematode morphotypes, 6 were identified to genus: a larval stage of the ascaridid Ophidascaris robertsi and adults of the chabertiid Cyclodontostomum purvisi, the physalopterid Physaloptera sp., the spirocercid Mastophorus muris, the strongyloidid Strongyloides venezuelensis, and the trichurid Trichuris sp. However, the remaining specimens were in poor condition and could not be fully identified. This study represents the first survey of the helminths of R. mordax, and all helminths are new host records. Ophidascarius robertsi and S. venezuelensis have not been reported previously from other Rattus spp. from Papua New Guinea. A bootstrap analysis estimated that 73% of possible species in the nematode component community of the helminth assemblage were recovered. The dominant species in the helminth assemblage was the spirocercid M. muris. It was the only species found in more than 1 individual, occurring in 50% of the hosts examined. By contrast, the dominant species in the assemblages of Rattus giluwensis, Rattus niobe, Rattus novaeguineae, Rattus steini, and Rattus verucundus were a heligmosomoid, a heligmonellid, an oxyurid, and a heterakid, or a spirurid, respectively. With the exception of R. giluwensis, heligmonellids were present at a low prevalence and intensities in each of the host species. Differences in habitat, diet, and behavior may account for the differences encountered between the helminth assemblages of R. mordax and R. niobe.