Nitrate-reducing ability, NO2−-reducing ability, and glutamic acid dehydrogenase levels were measured in two clones ofNodularia spumigenaMertens. Measurements with whole cells of both clones show that NO3−reduction is stimulated by NO3−, and that NO2−reduction is probably stimulated by NO2−. The NO3−-reducing system is stimulated by light and inhibited by NH4+. These controls and possible control over NH4+incorporation into amino acids are compared to systems operative in other organisms.Differences in growth and physiology of the clones in growth rate on N-free medium, growth inhibition by urea, and NO2−accumulation in the medium are discussed.