Abstract Sphagnum tundrae (sect. Squarrosa) is reported new to arctic North America and eastern Russia. This species was recently described from Svalbard, arctic Norway, and has until now not been reported from other areas. It is argued, that it reached Svalbard in early/middle Holocene by long distance chance dispersal through icerafting diaspores following main Arctic Ocean currents from arctic Eurasian sources. Supposedly closely related to S. teres, the distinctness of the gene pools of those two species is evaluated through isozyme analysis of a mixed population, as well as small samples from three other S. tundrae sites and two sites with S. teres populations from Svalbard. Fifteen putative enzyme loci were screened. Very little intraspecific genetic diversity was detected. Sphagnum tundrae was monomorphic throughout the four populations studied, whereas S. teres had three haplotypes. The mean genetic identity (Nei 1978) among populations of S. tundrae and S. teres in this sample from Svalbard was calculated as 0.525, which is in concordance with values obtained among other Sphagnum species. Sphagnum mirum is described as a new species from subarctic Alaska, differing in essential morphological characteristics from the related S. teres and S. tundrae. Morphological variation in mixed stands of S. mirum and S. teres from the type locality clearly demonstrates its specific distinctness in qualitative as well as quantitative characteristics.