This paper presents research on the metabolic activity of bacteria isolated from the surface of stems of the common reed and from the water in the pelagic zone of Moty Bay in Lake Jeziorak, in the presence of the following exogenous substrates: casein hydrolyzate, glucose, sodium succinate and sodium acetate, and in the presence of plant extracts obtained from the stems of the common reed and the leaves of floating pond-weed. It results from the research that epiphytic bacteria inhabiting the upper (near-surface) and lower (near-bottom) sections of the stems of the common reed were more metabolically active than planktonic bacteria. The best utilised substrate was casein hydrolyzate, and the most poorly was sodium acetate. In the presence of plant extracts, the metabolic activity of epiphytic bacteria was up to 13 times higher than the activity of planktonic bacteria. extracts