Eighteen bacterial stock cultures were examined for their ability to utilize NH4+ and NO3- simultaneously in a medium containing NH4NO3 with shaking using a test tube capped with a cotton stopper. Pseudomonas aeruginosa NBRC 12689 utilized I mg/ml of NH4NO3 most rapidly of the cultures tested. The bacterium could completely utilize 5 mg/ml of NH4NO3 within 3 d, 6 mg/ml of NH4Cl within 3 d, and 20 mg/ml of NaNO3 within 2 d under optimum conditions. The addition of Fe2+ to the NH4NO3 medium markedly promoted the utilization of the two ions. When the Pseudomonas strain utilized 5 mg/ml of NH4NO3 completely, the total nitrogen in the culture including its cells decreased to 41% of that of the NH4NO3 originally provided. GC-MS analysis showed that the removed nitrogen was probably denitrified. When the bacterium was incubated in the NH4NO3 medium with shaking in a vial sealed with a rubber stopper, N-2 accumulated, but not N2O at the final phase of cultivation. On the other hand, both N-2 and N2O were detected in the NaNO3 medium. We concluded that the bacterium removed NH4+ from NH4NO3 as a nitrogen source for its cell components, together with the denitrification of NO3- under controlled shaking conditions. In addition, NH4+ promoted the cell growth of the bacterium and denitrification to N-2, preventing the accumulation of N2O.