Restriction fragment length polymorphism (RFLP) was studied in restriction sites AvaII, BamHI, EcoRV, KpnI, HaeIII, and RsaI of the mitochondrial DNA (mtDNA) D-loop in populations of Old Believers (Starovery) and in Slavic migrants in northern Siberia. Frequencies of rare variants of all polymorphic sites studied were estimated. The results were compared with the published data on mtDNA polymorphism sites studied were estimated. The results were compared with the published data on mtDNA polymorphism in Russian populations of central and southern Russia and in other Eastern Slavic, Caucasoid, and Mongolian populations. Significance of interpopulation differences with respect to distributions of variants of polymorphism sites was estimated with the use of the chi2 test. The comparison did not reveal significant differences between any groups of Eastern Slavs, including Old Believers. However, they significantly differed from both Mongols and Europeans (P < 0.05). To date, stable estimates of the polymorphism level in most of the restriction sites have been obtained for the Russian population. Regarding the distribution of the restriction-site variants of mtDNA, Russians significantly differ from the total European population, as well as from Mongols. In the populations of Old Believers, the effect of isolation on the diversity of the mitochondrial genome was demonstrated.