An aerobic denitrifier was newly isolated from soil and its denitrification activity under different C/N (carbon/nitrogen) ratios was investigated. The isolate was identified as Pseudomonas putida AD-21 by biochemical studies and 16s rDNA sequencing analysis. P putida AD-21 tolerated oxygen levels of 5.0-6.0 mg/l. An (NH4)-N-15/(NO3)-N-14 analysis indicated that the nitrogen of NH4 was preferentially assimilated into the cell mass and that the nitrate removed could be considered an indication of bacterial denitrification efficiency. Increasing the C/N ratio increased the nitrate removal rates, whereas nitrogen assimilation into the cell mass was not affected. The optimal C/N ratio was 8 with a maximum nitrate removal rate of 254.6 mg/l/h and a nitrate removal efficiency of 95.9%. The results suggest that P putida AD-21 may be a good candidate for aerobic waste-water treatment.