Comparative analysis of the biology of insular and mainland populations has demonstrated a number of differences in characteristics of insular populations, termed the ‘island syndrome’. A subspecies of Arctic fox on the Commander Islands (Alopex lagopus semenovi on Mednyi Island) has been isolated for an evolutionarily significant time in small territories at the periphery of the species' range. The number of foxes on Mednyi had been observed to be very high since the islands were discovered in 1741, but a drastic decline in population density in the late 1970s, owing to mange, has left the population low. The aim of the study was to determine whether the Mednyi Arctic fox population exhibited the features expected in an isolated insular population, such as difference in body size, increased population density, larger social groups, lower tendency to disperse and lower fertility, and whether any behavioural changes were evolutionarily reinforced or were a temporal response to current ecological factors on the island. Eight predictions were identified based on the island syndrome, of which the Mednyi Arctic foxes conformed to seven. We suggest a new prediction, namely that the tendency for increased sex dimorphism in dispersal may also be a distinguishing feature of insular populations. All the features displayed by the insular population on Mednyi – conservative use of space, increased tendency to form complex groups, decreased fertility and dispersal – seemed to be preserved regardless of the currently comparatively low populations. Thus, although foxes have been below carrying capacity for at least 10 generations, island syndrome characteristics have persisted.