Metabolic activity of heterotrophic bacteria isolated from coastal Lake Łebsko was determined using the respirometric method. It has been shown that planktonic bacteria oxidized organic compounds with various intensity. Caseine hydrolyzate was the most intensively oxidized respiratory substrate, whereas cellobiose was oxidized the least intensely. In the studied lake, oxygen was taken up most actively by bacteria of the Flavobacterium-Cytophaga group and the genus Pseudomonas in their respiratory processes. The level of metabolic activity of bacteria from different parts of the lake was shown to differ. The oxygen uptake of bacterial mixtures was, as a rule, lower than the sum of oxygen uptake of single strains.