Incubation of sub-arctic soil and water microorganisms in the psychrophilic temperature range hindered neither growth nor efficiency of substrate utilization in batch systems. Nevertheless, no direct relationship was observed between the temperature of incubation and kinetic constants. However, the initial food:microorganism ratio appeared to have more influence on the cell growth rate than did any variation in temperature. Values obtained for the reaction rate coefficients for oxygen consumption in sub-arctic waters indicated no special oxygen requirement for the decomposition of sewage in these waters at low temperatures. Here again temperature seemed to have less of an effect on the rate of oxygen depletion by indigenous microorganisms than did the amount of nutrient present. Initial availability of nutrients was also seen to be important in the bacterial decomposition of cellulose. Cellulolysis was found to be difficult to initiate in a well-mixed, aqueous environment in which no supplemental carbon and energy source was present.