Namalycastis tiriteae, a new species of polychaete (Nereidae: Namanereinae) is described from a freshwater stream in New Zealand. Morphologically it possesses some features intermediate between Namalycastis Hartman and Namanereis Chamberlin as defined by Hartman (1959), but it is placed in the former on the basis of parapodial structure. Notes on physical and chemical conditions in the worm's habitat are also given.