The objective of this research was to obtain data on the metabolic reaction rates of the microorganisms indigenous to the cold environments of the arctic and sub-arctic in order to evaluate the natural abilities of the freshwater streams and lakes of Alaska to assimilate the wastes discharged into them. Microorganisms capable of growth even at subzero temperatures have long been known; however, most have consistently fared better at higher temperatures, usually above 20° C. Much of the work done with the biological oxidation of wastes at low temperatures has been with organisms of this type : mesophilic organisms which are able to survive at low temperatures but which are metabolically much more active in the temperature range from 20 to 45° C. Such organisms might be labeled "cold-tolerant," but they are probably biochemically quite different from the truly "cold-loving," or psychrophilic, microorganisms which are able not only to survive but also to thrive at temperatures below 20° C and which, in fact, find temperatures much higher than 25° C intolerable.