The article is devoted to the study of bacteria in the system of technical water supply of Balakovo nuclear power plant (BNPP). The work was carried out in 2017. The samples were taken at the following sites: backup diesel generator of the 3rd unit (rode’s-3), pumping station of feeding pond-cooler (PSFPC), coastal pump station of unit 3 (CPS-U3), coastal pumping station of power unit № 4 (CPSP-U4), turbine hall of power unit № 3 (TH-PU3), turbine hall of power unit № 4 (TH-PU4). The results showed that the metal structures (BNPP) formed a microbiological film, in which were found saprophytic chemoorganotrophic mesophilic aerobic and facultative-anaerobic bacteria of 5 genera (Aeromonas, Bacillus, Dietzia, Rheinheimera, Vibrio) as well as chemolithotrophic bacteria from the group of iron bacteria being in the association. The quantitative indices of species isolated from biofilm ranged from 102 to 108 microbial cells in 1 ml. The isolated microorganisms are represented by species of bacteria widespread in natural water and soil ecological systems, however some species of isolated genera of bacteria are opportunistic bacteria and can excite stimulate infectious diseases in humans and animals. The bacteria detection in water biotopes might be associated with temperature values that are favourable for their growth.