SUMMARY: Increased oxygen tension (pO2) caused increased respiration by excised soybean nodales of all ages. The increase took place in two steps, the first maximum occurring at about 50% O2 and the second at 90–100% O2 for actively nitrogen-fixing nodules. With increasing nodule age the first maximum occurred at decreasing pO2 until, when fixation ceased at about 6 weeks, this maximum had disappeared. This effect was more marked at 30° than at 23°. The respiration of bacteroids increased with increasing pO2 with a maximum at 2–3% O2; the curve indicated a simple saturation of the terminal respiratory pathway with O2. Increased pO2 raised nitrogen fixation by excised nodules to a maximum which corresponded to the first maximum of the respiratory response to raised pO2; higher pO2 than this decreased nitrogen fixation. Sliced nodules showed the same effect but the stimulation of fixation at the lower pO2 levels was not as great as with intact nodules. The Michaelis constant (Km) for nitrogen fixation by intact excised nodules was relatively unaffected by pO2 until this reached the pO2 for maximum fixation when the Km rose sharply. At external pO2 of 80%, oxygen was shown to be a competitive inhibitor of nitrogen fixation.