Abstract: When A. cylindrica was grown under the condition of N2-deficiency, an increase in N2-fixing activity and a decrease in oxygen sensitivity were found simultaneously. The acetylene reduction was inhibited by 50% by 0.5 atm. partial pressure of oxygen (pO2) in the N2-deficient algal cells, and by 0.35 atm. pO2 in the N2-sunfieient algal cells. It indicates that the enhanced nitrogenase activity is possibly due to an increase in respiratory (Vconsnmptiion of the alga.Further research suggests that there is close relationship between respiration and nitrogenase activity. "When the alga grew under such conditions as N2-sufficiency, N2-deficiency and light induced nitrogenase synthesis, the N2-fixing activity varied with respiration rate correspondingly. Our results support the concept that the main means of nitrogenase against O2 damage is respiration protection.