A denitrifying strain of phototrophic purple nonsulfur bacterium, Rhodobacter sphaeroides NII2, which was isolated from, Lake Nakaumi, was characterized for nitrogen and carbon metabolisms. This strain grew well utilizing (NH_4)_2HP0_4 as introgen source, but was incapable of ultilizing KN0_3 as nitrogen source under anaerobic condition. On the other hand KN0_3 was reduced to N_2 through several intermediary metabolites under both light and dark culture conditions. The denitrification activities of dark-grown cells were 1.5-fold higher than those of light-grown cells, indicating that the denitrification of the strain NII2 might be repressed by light irradiation. In addition, light-grown cells produced large amounts of alanin derived from pyruvate, the final product of glycolytic pathway. The amounts of alanin decreased with increasing of KN0_3 added. In contrast, dark-grown cells produced large amounts of glutamate derived from 2-oxo-glutarate, an intermediate of TCA cycle. These results indicate that light irradiation and nitrate respiration have a close relation to nitrogen and carbon metabolisms in the bacterium.