Studying the development of such an important group as cyanobacteria is an integral part of environmental monitoring of the Volga-Caspian region. Based on long-term data, a comparative description of the current state of the group of cyanobacteria of the Volga-Caspian region is indicated. The degree of similarity of marine and river species of the group of cyanobacteria for sections of the Volga River, Northern and Middle Caspian Sea was determined. Calculation of the percentage contribution of cyanobacteria to the total composition of phytoplankton in the studied water areas made it possible to estimate their importance. The dominant species among the studied group of microalgae and its quantitative characteristics in the considered water areas were identifi ed. During periods of growth of cyanobacterial biomass, a large proportion of it consists of large colonies of cells (species of the genus Microcystis, Gomphosphaeria), which become available in the form of detritus as food for organisms of most links in the trophic network after death, which has a positive eff ect on the development of these organisms. Changes in the indicators of the group of cyanobacteria over a number of years and relative to long-term data indicate their quantitative decline, however, the species diversity of the group remains at a fairly high level. The purpose of the work is to establish the qualitative diversity of cyanobacteria in the river and sea and to clarify their role in the phytoplankton of the Northern and Middle Caspian Sea and the lower part of the Volga River.