Woodland (Rangifer tarandus caribou) and barren ground (R. t. groenlandicum) caribou are reported for the first time as hosts of Parelaphostrongylus andersoni, greatly extending the known geographic range of this muscle nematode. Up to 56% of caribou in the Beverly herd, central Northwest Territories, passed dorsal-spined first-stage larvae in their feces. Animals less than 3 years old were more frequently infected and passed greater numbers of larvae than older animals. Larval output in winter and spring did not differ. Adults of the four elaphostrongyline nematodes known to occur in caribou are distinguished by their location in the host, the size of the worms, and the size and morphology of the male copulatory structures. There is an urgent need for tested and improved methods of differentiating larvae of P. andersoni and other elaphostrongyline nematodes from those of P. tenuis that cause neurologic disease in various North American cervids. Demonstration of the occurrence of P. andersoni in Rangifer sp. in North America raises the possibility that it originated in Eurasian cervids and may still occur in the Old World.