Surveys for macroinvertebrates occurring in 20 small attached lakes (naiko) adjacent to Lake Biwa, central Japan, conducted in 2001–2002 yielded 44 taxa of aquatic oligochaetes (Annelida, Clitellata). Descriptions and taxonomic remarks are provided for nine species: Aulophorus furcatus, A. tonkinensis, A. flabelliger, Pristina biserrata, P. proboscidea, Ilyodrilus templetoni, Allonais pectinata, Stylaria fossularis, and Dero dorsalis; the former six species of these are new to Japan. Stylaria fossularis is regarded as a distinct species, distinguished from the congener S. lacustris, on the basis of prostomium morphology and geographical distribution. No distinctive oligochaete fauna can be seen in the individual naiko, but as a collective body they maintain a rich fauna. This fauna was richer in naidines, while poorer in other groups of oligochaetes than those in Lake Biwa, and basically can be regarded as an extension of the fauna of littoral Lake Biwa. Epiphytic oligochaetes were more diverse and more abundant in the naiko than those in littoral Lake Biwa, probably because of higher temperatures, denser aquatic vegetation, and higher primary production. Summer oligochaete assemblages in the naiko were predominated by Stylaria fossularis and include several species characteristic of subtropical and tropical Asia, suggesting a close relationship to the fauna of the Indo-Malayan zoogeographical subregion.