The sequence of the first hypervariable segment (HVS-I) of mitochondrial DNA (mtDNA) was determined in 251 individuals from three eastern Slavonic populations, two Russian and one Belorussian. Within HVS-I, 78 polymorphic positions were revealed. Within-population diversity of HVS-I varies slightly among three samples; its estimates do not differ strongly from those for European populations. Haplotype diversity for three populations calculated in this study is 0.949; mean pairwise differences estimate is 3.59. To assign mtDNA sequences to major phylogenetic clusters, haplogroup-specific restriction polymorphisms were selectively typed in most samples. The haplogroup distribution in the total Eastern Slavonic sample is similar to that reported for the European sample. However, the separate consideration of three Slavonic samples reveals the complicated structure of the mitochondrial gene pool in the Eastern European area. Data of this study support the proposed model of the origin of modern Eastern Slavs, which implies the admixture of ancient Slavonic tribes with pre-Slavonic populations of Eastern Europe. These data should contribute to general studies of mitochondrial DNA variations in Europe.