Genetic and some ecophysiological traits of mussels collected in the European Arctic, up to their northeastern distribution limit in the Barents Sea, were studied and compared with traits of mussels from the Mediterranean, Atlantic and Baltic. The genetic traits of these populations we-re analysed by isoenzyme electrophoresis on seven loci in order to assess the Mytilus complex to which populations in the Arctic region belong. Ecophysiological variables, the weight-index and glycogen were analysed to assess the physiological fitness of the populations. Three distinct groups were recognised: (1) Mytilus (edulis) galloprovincialis in the Mediterranean and Spain, (2) M. (edulis) edulis along the Atlantic coast from the Netherlands northwards into Russia, and (3) the Baltic Mytilus (edulis) trossulus. The mussels from populations in the Russian Arctic all belong to the Atlantic Mytilus (edulis) edulis group. The genetic variability and ecophysiological measures indicated that the sub-Arctic White Sea mussel populations have a relatively lower performance capacity, whereas those in the Arctic at the edge of their northern distribution showed a surprisingly strong performance.