Timing of flowering in six natural populations (two insular and four continental) of Arabidopsis thaliana (L.), occupying northern limits of the species range (Karelia), was studied. Average fitness of these populations was estimate as well. It was revealed that Karelian populations are mainly represented by late=flowering forms. However, diversity in the timing of flowering and different reaction to vernalization indicates that there is polymorphism of genes which control this trait in Karelian populations. Perhaps it is the original adaptive strategy of A. thaliana in the unstable environment at the northern limits of the species range. It was demonstrated that adaptation of Karelian populations is facilitated by the high viability potential and high level of fertility variation. In addition to that, insular populations turned out to be better adapted than continental ones. Evidently, intensification of inbreeding, as a result of insular populations isolation, does not lead to any significant fitness decrease in the highly selfing species A. thaliana.