The biological fixation of dinitrogen is the most important way to access of N to organisms, this process requires a fairly high proportion of the ATP; which is generated in the course of respiratory electron transport reactions with O2 as electron acceptor. The Nitrogenase enzyme complex (the nitrogen. fixing enzyme) is sensitive to O2, that irreversible inactivates the enzyme. Diazotrophs must employ mechanisms which, on the other hand, permit the supply of O2 required for energy regeneration and protect Nase from the deleterious effect of O2. They have developed several strategies for limiting O2 access to Nase: 1).--It could avoid O2 and live in environments which are permanently anaerobic, 2).--Alternatively, it could generate a physical barrier around its Nase and in this way prevent O2 from diffusing to the enzyme, 3).--The microorganism could, by its metabolism, reduce the concentration of O2 within the vicinity of Nasa, 4).--They could modify its Nasa in such manner as to render it resistant to inactivation by O2 (conformational protection). 5).--Finally, the microorganism could simply balance Nasa inactivation with the synthesis of new enzyme. In this article we examine the antipathy between Nasa and O2, particularly with strict aerobic and photosynthetic microorganisms.