The addition of NH4Cl at concentrations of more than 1 mM completely inhibited nitrogenase-dependent hydrogen evolution using 1 mM succinate as a substrate in the marine photosynthetic bacterium Rhodopseudomonas sp. strain W-1S. However, cells could derepress nitrogenase within 6 h in the presence of NH4Cl. The inhibition by 1 mM NH4Cl was removed by increasing the concentration of the substrate for hydrogen evolution. The addition of L-methionine-D,L-sulfoximine (MSX), an inhibitor of glutamine synthetase (GS), also removed the inhibition by 1 mM NH4Cl. These results indicated that under an argon gas phase, nitrogenase-dependent hydrogen evolution is inhibited by ATP-consuming GS activity in the presence of NH4Cl. We isolated a mutant strain, CR-8, which is capable of active hydrogen evolution in the presence of 1 mM NH4Cl.