The species diversity of mosses is studied in mires of south-western Arkhangelsk Region and adjacent areas of the Vologda Region and the Republic of Karelia. The contributions of different species to plant associations is analyzed in detail, taking into account their dominance levels, percent cover, and the number of associations they were found in. Mires of the region were found to harbor 87 moss species. Sphagnum mosses (Sphagnopsida) are the most significant: of the 28 sphagnum species, 19 were dominants. The coenotic role of true mosses (Polytrichopsida, Bryopsida) is much weaker: 17 of the 59 registered species acted as dominants, but only in a very limited number of plant communities. Six species listed in the Red Data Book of the Arkhangelsk Region (2008) and one species, Meesia hexasticha, new for European Russia were identified.