Abstract
A new naidid oligochaete, Nais kisui, is described from oligohaline habitats in northern Japan. This new species is similar to N. borutzkii Sokolskaya, 1964, a species known from Far Eastern Russia, in that both species have sigmoid needle chaetae in the dorsal bundles and protrusible projections of the spermathecal ducts, which in both species are set in deep chambers created by infoldings of the body wall. However, N. kisui differs from N. borutzkii in having inconspicuous male efferent ducts, each with a conical penis that opens into a median male bursa.
