Studies on the oxygen requirements of aerobic psychrotolerant bacteria showed that the oxygen concentration of the atmosphere can be lowered to 2% without noticeably inhibiting growth rate. Tests were made with meat-spoilage strains of Achromobacter and Pseudomonas in nutrient media sparged with gases containing from 0.1 to 100% oxygen. Reduction of overall growth rate with reduction in oxygen concentration below 2% was mainly the result of an increase in the lag period. At an oxygen concentration of 0.5% or lower, however, cell generation time during the logarithmic phase was increased and cell yield at the stationary phase was reduced. The increased lag period could be eliminated by using an inoculum adapted to the oxygen concentration under test. Growth did not occur in the absence of oxygen. Oxygen concentrations greater than 21% (air) had a negligible effect on Pseudomonas, but inhibited the growth rate of Achromobacter. The results indicate that insufficient oxygen is not a major cause of the failure of these bacteria to grow rapidly in vacuum packaged meats.