The study was conducted on peat-muck soils at different states of secondary transformation. The main purpose of the research was to determine changes in quantity of the following bacteria: copiotrophics, oligotrophics, aerobic and anaerobic sporeforming, ammonifying, nitrate reducing and respiration and dehydrogenase activity. Respiration and dehydrogenase activity and quantity of microorganisms decreased in the soil samples at the higher stage of secondary transformation (Mt III). Intensity of respiration and dehydrogenase activity was modified by the addition of water solution of substrates.