Thomsen’s (TM) and Becker’s (BM) myotonias are nondystrophic myotonias. At present, 150 mutations in the CLCN1 gene, which results in the development of TM and BM, have been described. The c.2680C>T (p.Arg894*) is the most common mutation. In the Northern Scandinavian countries, the population frequency of this mutation is 0.87%, while in the Russian Federation, it is equal to 1.2% (this study). Based on the results of a molecular-genetic analysis of CLCN1 gene in patients with nondystrophic myotonias, the calculated frequency of TM and BM in Russia is 1: 8165 and 1: 710, respectively. We have conducted haplotype analysis using microsatellite markers and intragene SNP, which has shown that the prevalence of p.Arg894* mutation in Russia results from the founder effect, and the time of its scattering is 3680 ± 1240 years.