Many studies have been made on the influence of oxygen tension upon the growth and activity of various organisms (Amberson, 1928, Hyman, 1929), but relatively few measurements of oxygen consumption have been made. Although it is generally agreed that most bacteria have definite oxygen optima for multiplication and for certain physiological activities, no generalizations can be made from the fragmentary literature on the influence of oxygen tension upon the rate at which oxygen is consumed by respiring cells. Escherichia coli was found by Stephenson and Whetham (1924) to consume much more oxygen in an atmosphere of pure oxygen than in one of air. Wohlfeil (1930) also noted that the rate of oxygen consumption by this organism is proportional to the oxygen tension. Similarly Novy and Soule (1925) found that the maximum oxygen consumption by Mycobacterium tuberculosis occurred when it was cultivated in an atmosphere containing 40 to 50 per cent4 oxygen; growth and oxygen consumption decreasing progressively below this value. According to Georgi and Wilson (1933) the respiration of the Rhizobia increases with 1 This work was made possible by grants from the Brittingham Trust Fund and the Wisconsin Alumini Research Foundation.