This study concerns developmental intensity and metabolic activity of heterotrophic bacteria isolated from the water of the littoral and pelagial zones of the polyhumic lake Stęgwica. The experiments were carried out with humic substances and [³H]-glucose in media with pH 6.0 and 8.0. It was found that the development and metabolic activity of most strains in the presence of humic substances were higher in the medium with pH 8.0 than in that with pH 6.0. It was found that humic substances in acid medium had an inhibitory effect on the development and metabolic activity of 90% of the strains of bacteria isolated from the littoral zone and of 50% of the strains isolated from the pelagial zone.