Abstract: When photoautotrophically-grown Anabaena culture was transferred to darkness/air, the reduction rate of nitrogen-fixing activity was proportional to oxygen concentration. When dark-treated culture (in air for 12 hrs) was reilluminated, the recovery of activity was depressed by chloramphenicol, NH4 or by 38—40℃/air. This implies that the preformed nitrogenase was inactivated by atmo-spheric oxygen in darkness. When Anabaena cells were exposed to different oxygen tensions for a short time (30 min) in light or in darkness, the decline of activity in darkness was much greater than that in light. Ascorbate could alleviate the inactivation at higher oxygen content, whereas high temperature may enhance the oxygen sensitivity of nitrogenase. Treatment of cells with 100% O2 for up to 120 min did not result in further inactivation, nor did it show any recovery of the activity. Therefore, no comformational protection has been observed. Relationship between oxygen protection and environmental conditions is discussed.