Dehydrogenase activity was studied in the sediments of three lakes of the Iława Lakeland (Jeziorak, Mały Jeziorak and Jasne) with reference to the type of lake, the season of the year, the situation of the sediments, pH and temperature. The highest activity was found in the sediments of the eutrophic lake Jeziorak sampled during the summer from places in the vicinity of a village. The lowest activity was exhibited by sediments sampled in the autumn in the lakes Jasne and Mały Jeziorak. No correlation was found between dehydrogenase activity and the number of bacteria in the sediments. The highest dehydrogenase activity was found at 20 degrees and pH 7.0 and 8.0--8.25.