Studies were conducted on the northeastern shore of Lake Ladoga in 1966-2004 (Russian Karelia). The mechanisms controlling the density and structure of the bank vole population and the relative significance of exo- and endogenous factors influencing the species abundance were determined using multivariate analysis. The intensity of reproduction in voles in spring and early summer, as well as their survival rate in autumn-winter, and total abundance were found to be mainly related to the demographic situation that had developed in the population in the autumn of the preceding year and to the wintering conditions. In the breeding season, reproduction and abundance of voles are determined primarily by infrapopulational factors. At other times, the population is controlled chiefly by external effects. Exogenous factors determine the upper limit of the optimal population density under given conditions, while endogenous mechanisms bring the species numbers to the level corresponding to the conditions.