The brackish water plathelminth fauna of Alaska is compared to European and Canadian brackish water biotopes. 37 species are presented including descriptions of 18 new species. 13 species (35% of the total number found in Alaska) also occur in Europe and 6 (16 %) in SE Canada. Most of the common species are confined to brackish waters. Coronhelmis lutheri, Halammovortex nigrifrons and 3 marine species are common to Alaska, Canada, and Europe. Maehrenthalia americana n. sp. occurs in Canada and Alaska but is unknown from Europe. Results corroborate the hypothesis of the existence of a boreal to subarctic brackish water community of Plathelminthes with a circumpolar distribution.