Relationships between members of the parasitic system formed by the taiga tick Ixodes persulcatus (Schulze, 1930) at the preimaginal developmental stages, Borrelia garinii and small mammals were studied. Transovarial transmission of B. garinii in the taiga tick was not found. In natural biotopes of northwestern Russia, small mammals that were hosts of the preimaginal stages of Ixodes persulcatus included Myodes glareolus (Schreber, 1780) (52-53%), Sorex araneus Linnaeus, 1758 (33-34%), and Apodemus uralensis (Pallas, 1811) (14%). Larvae and nymphs were most numerous on hosts at the beginning of the season, in May and June. Subsequently, their number decreased by 5-10 times and slightly increased in September. In populations, up to 31.9% of M. glareolus individuals and 20.5% of A. uralensis individuals were infected with B. garinii. The aggregated type of distribution of taiga tick larvae and nymphs on hosts has been revealed. Infection of the main part of hungry larvae and nymphs occurred at the beginning of the activity season after wintering when feeding on overwintered infected hosts. This period is the key in circulation (exchange between vectors and reservoir hosts) and the preservation of B. garinii in the natural focus of borreliosis in northwestern Russia. Vertical transmission of B. garinii is possible from larvae and nymphs fed at the beginning of the season to nymphs and adult ticks that had molted in August-September, respectively. Horizontal transmission of the pathogen from small mammals, especially the bank vole, to larvae and nymphs, and vice versa, ensures the circulation and persistence of B. garinii in natural foci. The question of the preservation of the pathogen in overwintering preimaginal stages of ticks requires further research.