During a parasite survey of galaxiid fishes (Galaxiidae) from Patagonian Andean lakes, a new species of nematode, Hedruris suttonae n. sp. was collected from the stomach of the native Galaxias maculatus (Jenyns) and G. platei (Steindachner). Specimens were examined by light and scanning electron microscopy, especially head morphology, female caudal prehensile structure, and distribution of spines. The new species is distinguished by body and tail size, morphology and size of spicules, the arrangement of caudal papillae in the male, the female caudal hook, and size of eggs. Hyalella patagonica (Ortmann), a Neotropical species of Amphipoda, is reported as its natural intermediate host. Data regarding prevalence and mean intensity in the intermediate and definitive hosts are included. The diet and habitat of the hosts, the percentage of gravid females, the high values of prevalence, and mean intensity in galaxiid fishes, as well as the wide distribution of H. suttonae, collectively indicate that, in these oligotrophic Andean lakes, G. maculatus and G. platei are true definitive hosts of this nematode.