It has been found that during algal blooms in an eutrophic lake occurs elimination and selection of bacteria. In this period a strong decrease in number of bacteria in the water was observed. The effect of the antibacterial substances produced by algae on the growth and respiration of bacteria isolated from the lake was investigated. The bloom forming algae were found to produce substances inhibiting the growth and respiration of Gram positive bacteria. Gram negative rods were resistant to the action of these substances. The algal substances inhibited also degradation of organic matter in the lake but only in the euphotic zone.