Namalycastis tiriteae, a new species of polychaete (Nereidae: Namanereinae) is described from a freshwater stream in New Zealand. Morphologically it possesses some features intermediate betweenNamalycastisHartman andNamanereisChamberlin as denned 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.