Microbial communities of alluvial soils in the facies on islands in the Selenga estuary and near Stepnoi Dvorets and Istomino Settlements were studied. The distribution of the CFU-dominant microorganisms ( Bacillus and Pseudomonas + Arthrobacter ) has been revealed in the studied island soils. In addition, actinomycetes Streptomyces were found among the dominants in soils of the near-terrace floodplain. The quantitative distribution of microorganisms across soil layers was demonstrated. The effect of ecological factors in formation of microbial communities on the islands and central floodplain regions was revealed.