This paper presents the results of research on the numbers of planktonic bacteria in Lake Chełmżyńskie, their developmental dynamics and their physiological properties. It was found that the total number of plank-tonic bacteria (TNB) was between 0.01 to 34.90 x 107 cells . ml-1 while the number of heterotrophic bacteria (TVC 22°C) in the water of the research sites in the lake varied between 0.17 to 42.06 x 102 cells . ml-1. The maximum number of bacteria was found in summer, and the minimum in autumn. In the part of the lake near the town (sites I-VI) a distinctly greater number of planktonic bacteria was found in spring and autumn than in the part far from the town (sites VII- IX). Based on Korsh’s bacteriological “Q” index (Q > 1000) the water of Lake Chełmżyńskie is rated as clean. Among the planktonic bacteria of Lake Chełmżyńskie, Gram-negative rods were dominant, and the majority were slowly growing strains. The most numerous among planktonic bacteria were strains that hydrolyse fat, protein, deoxyribonucleic acid and starch. The least numerous were chitinolytic, nitrifying and ureolytic bacteria. Bacteria belonged to genera Flavobac-terium, Aeromonas and Alcaligenes dominated in all examined periods.