The distribution of alleles with various CTG-repeat numbers was studied and the haplotypes for polymorphic sites HhaI and HinfI of mouse muscle protein kinase (DMPK) were analyzed in inhabitants of northwestern Russia and in patients with myotonic dystrophy (90 and 18 chromosomes, respectively). Twelve normal alleles with the triplet-repeat number from 5 to 24 were identified and the alleles with five (42.5%) and 11-13 (37%) repeats were found to be predominant. The bimodal distribution revealed is similar to those described earlier for other populations, however, the frequencies of individual alleles differed from those in populations of Europe and Central Russia. No significant differences in frequencies of CTG alleles were found in 32 normal chromosomes involved in compounds with the mutant chromosomes (i.e., in patients with myotonic dystrophy) as compared to their frequencies in the population. However, almost all mutant chromosomes (16 of 18) had the same haplotype for intragenic polymorphic sites: HhaI-; HinfI+. This haplotype was also inherent in 91% of all chromosomes with CTG5 and all chromosomes with a CTG number more than 15. Possible evolution of chromosomes with different numbers of triplet repeats mediating their expansion and impairing the function are discussed.