A thermotolerant bacterium, identified as Bacillus licheniformis, completely utilized 0.1% (w/v) NH4NO3 at 30 and 50 degrees C under aerobic condition. The addition of 0.5 mM Fe2+ to the NH4NO3 medium markedly promoted the utilization of NH4+ and NO3-. At 50 degrees C, of total nitrogen originally provided, 24% was taken up into the cells and 20% remained in the culture supernatant. Residual nitrogen (56%) was probably removed into the atmosphere. The cell extracts contained enzymes involved in denitrification. GC-MS demonstrated that NH415 NO3 had been converted to (N2O)-N-15. These results indicate that the strain has denitrification ability under aerobic condition.