This Bulletin is concerned with the benthic errantiate polychaetes (Annelida) of British Columbia and Washington, the central and best-known section of the southern part of the Oregonian-Aleutian biogeographic region. The principal external diagnostic characters of all the families of benthic errantiate polychaetes are presented in a tabulated scheme. Keys to all the genera from the continental shelves of the cold-temperate North Pacific Ocean are provided under the relevant family. In contrast, the species keys treat only those species recorded, landward of the 200-m isobath, from British Columbia and Washington and an additional 22 species which may be expected to occur there. A checklist of the 195 species known from British Columbia and Washington is provided; names in boldface indicate that the authors have seen specimens from British Columbia or Washington. The diagnostic characters àre presented in the specific keys, as well as in the generic keys, in a series of alternative choices. Supplemental characters for certain species, however, are added to preclude species, not previously recorded from British Columbia and Washington but known from elsewhere in the cold-temperate North Pacific, from fitting the key accidentally. References are cited below the specific name if the species was not treated in Canadian Pacific Fauna (Berkeley and Berkeley 1948) or if the description was inadequate or the species was misidentified therein. About 330 detailed figures are given. Brief instructions for collection, fixation, identification, and use of the key, as well as a glossary and an index to the scientific names, are provided. Onuphis longibranchiata Berkeley is referred to Diopatra ornata Moore. A diagnosis of Bergstroemia Banse, a new subgenus of Eulalia Savigny s.1. (Phyllodocidae), is added.