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The length of 469 Borreliae burgdorferi s.I. from the Ixodes ricinus and I. persulcatus images collected in the Moscow Region, that of 5433 B. burgdorferi s.s from the I. persulcatus nymphs and images cultured at a laboratory, and B. burdorferi s.s. grown on the BSK-II (1 and 10 passages) were measured. There was a wide range of variations in the length of specimens (3-74 microns) and in those of this group average sizes (10.7-24.8 microns). The lengths of Borelliae from natural and laboratory ticks after their molt were 17-18 microns. When the ticks were kept in the refrigerator as long as 1-2 years, the length of Borreliae decreased to 10.7-10.9 microns, upon multiple (10) passages on the BSK-II medium, their lengths increased to 24.8 microns (the differences being significant). When the length of Borreliae reduced due to their keeping in the refrigerator, their pathogenicity for albino mice diminished. This disappeared after multiple BSK-II medium passages. It is suggested that the length of Borreliae may serve as a marker of their pathogenicity.