The polymorphism of mtDNA was examined in populations of Old Believers (n = 104) and Russians from Novosibirsk oblast (n = 270). Most of the haplogroups identified belonged to West Eurasian lineages. The frequencies of these haplogroups constituted 96.6% in Russians from Novosibirsk and 93.2% in Old Believers from Tyumen oblast. The populations examined were characterized by a high mtDNA diversity level (h = 0.98) compared to other population samples of Russians from Russia. Among the West Eurasian haplogroups, the most common (a frequency of more than 10%) were haplogroups H, U, J, and T, the proportion of which constituted 77.9% in Old Believers and 83.1% in Russians from Novosibirsk. The Mongoloid admixture in Russians (3.3%) and Old Believers (6.7%) was represented by haplogroups A, D, Z, and C, D, M*, respectively. Statistically significant differences (P