Studies were undertaken to determine the changes in the soil microflora during the decomposition of plant residues. The methods used include the determination of bacterial numbers, dehydrogenase activity, and oxygen uptake with selected substrates (ethanol, amino acids, organic acids). Tests were conducted at varying intervals during a 66-day experimental period. Maximum activity was observed in all supplemented soils during the early stages of decomposition (5–10 days) followed by a rapid decline. During the first 10 days a correlation was found between numbers of microorganisms and the respiratory activity (oxygen uptake) of the soils. Analysis of the data from this period indicated that the increased metabolic activity of the soil was due partially to a preferential stimulation of the organisms and not to the effects of increased numbers alone. Nutritional grouping of bacteria isolated from the flax supplemented soil at 0 and after 6 days of decomposition indicated a shift in the microbial equilibrium, towards a higher proportion of bacteria with relatively simple nutritional requirements.