Conditions for continuous culture of Escherichia coli K-12 His- Thi- under glucose limitation were established. Both the capacity for respiration, at D greater than 0.2/h, and specific activity of superoxide dismutase increased as a function of specific growth rate, whereas peroxidase and catalase were either invariant with or inversely related to this growth rate. The abrupt increase in the availability of glucose, as a means of elevating the growth rate, was followed by an increase in superoxide dismutase, which reached a plateau before there was a significant increase in the growth rate. Thus, an increase in superoxide dismutase appeared to be a prerequisite for an increase in the rate of growth. Cells that had higher levels of superoxide dismutase, because of varying specific growth rates, were more resistant to the toxicity of hyperbaric oxygen. Superoxide dismutase thus behaved like an essential defense against the toxicity of oxygen. Sensitivity towards streptonigrin increased with specific growth rate in the range of 0.09 to 0.25/h but decreased with further increases in the growth rate. Since this antibiotic has been shown to shunt electrons to oxygen, with concomitant production of O2-, these results indicated a progressive deficiency of reducing power at growth rates below 0.25/h and a surfeit of reducing power with progressively greater protection against O2- by superoxide dismutase at growth rates greater than 0.25/h.