The originally Pacific temperate blue mussel Mytilus trossulus and its Atlantic congener M. edulis are closely related, widespread benthic species. They can be routinely distinguished by molecular markers while reliable diagnostic morphological markers are lacking. Whenever areas of the two species overlap they fall into introgressive hybridization Historically in Northern Europe populations of M. trossulus were known to be confined to the Baltic Sea. Recently M. trossulus was found in some other regions of the Northern Europe, in particular in the White Sea it coexists with M. edulis. Here we use a set of four semi-diagnostic allozyme loci to perform a detailed survey of Mytilus populations in the Kandalaksha Bay of the White Sea. We use genetic data to map the distribution of M. trossulus and M. edulis in the region (i); to evaluate the extent of hybridization between the two species (ii); to determine the micro-biotopic (substrate) preferences of genotypes; vi) study size distributions of genotypes. We als