Genetic diversity of T4-like bacteriophages of the family Myoviridae from Selenga River was analyzed using major capsid protein gene g23. As a result, high diversity of T4-like phages, closely related to phages of eutrophic habitats, was revealed in the Selenga River. The phylogenetic relationship of the significant identified phage sequences isolated from E. coli (Escherichia phage 121Q) assumes that these sequences belong to viruses infecting pathogenic or potentially pathogenic bacteria. This attests to indirect evidence of the presence of pathogenic organisms in the river.