The moderately halophilic, halotolerant bacterium Ba 1 can utilize either nitrate or ammonium as a nitrogen source, but cannot use nitrate as an electron acceptor for anaerobic respiration. When both nitrate and ammonium are present in the growth medium, nitrate is taken up only after all the ammonium has been exhausted. Nitrate reductase of this bacterium is associated with the respiratory electron transport chain on the cytoplasmic membrane, and is able to accept electrons from NADH, succinate and malate, but not from NADPH. Chlorate is a competitive inhibitor of the enzymes which is also inhibited by antimycin A, cyanide, azide and p-chloromercuribenzoate. Cellular activities of the enzyme are not affected by the type of nitrogen source used by the cells. Nitrite reductase is a soluble enzyme, its preferred physiological substrate is ferredoxin, and its synthesis is repressed by ammonium and induced by nitrate. Glutamine synthetase is repressed by ammonium, whereas glutamate dehydrogenase is induced by it; activity levels of glutamate synthase are not affected by the nitrogen source in the growth medium. Glutamine synthetase activity is inhibited by amino acids and by magnesium ions. The ratio of activity + Mg 2+ − Mg 2+ is 0.074 for nitrate-grown cells, and 0.42 for cells grown with ammonium alone or plus nitrate.