Studies were carried out on metabolic activity of microorganisms inhabiting the bottom sediments of three lakes of different trophy. For respirometric studies the Warburg apparatus was used. Oxygen uptake was different in various sediment samples supplemented with organic substances, depending on the source and time of sampling. Cystein, aspartic acid and intermediates of the tricarboxylic acids cycle were oxidized most readily. Carbohydrates and especially maltose, celobiose, galactose, and glucose were metabolized less willingly.