The N-fixing system in the blue-green alga, N. muscorum, was investigated by total-N expts. based on Kjeldahl analyses and in isotopic tracer expts. using the heavy nitrogen isotope. The properties of the system in N. muscorum resemble those previously observed in the Azoto-bacter and the symbiotic system of Trifolium plus Rhizobium. The Michaelis constants are reasonably close to one another, and all the systems are sensitive to low concs. of CO and are specifically inhibited by H2.