Under an atmosphere containing 97% Ar and 3% CO₂ or containing 100% Ar, the nitrogenase activity of Anabaena 7120 was much higher or lower, respectively, than that in air. When the treatment was done in the above listed atmosphere considerable variation in the NH₄⁺-derepression of nitrogen fixation in Anabaena occurred. Moreover, the response of NH₄⁺-derepression of nitrogen fixation in Anobaena to physiological conditions differed. When algae was incubated in O₂ or N₂, Respectively, the extent of inhibition of NH₄⁺-derepression in Anobaena with a lower nitrogen-fixing activity was greater than that in one with a higher activity. A great acceleration of NH₄⁺-derepression of nitrogen fixation by hydrogen in Anabaena with a higher nitrogen-fixing activity was observed. The rate of NH₄⁺-derepr;.ssion of nitrogen fixation in Anabaena with a higher nitrogen-fixing activity was significantly faster when the hydrogen was added with oxygen or added only with exogenous carbonhydrates, such as sucrose or pyruvate, to the test system. Under weak light or in the presence of phtosynthetic inhibitors in the reaction system, the NH₄⁺-derepression of nitrogen fixation was more pronounced in Anabaena with a lower nitrogen-fixing activity.