We studied the effect of N-oxide of N, N-dimethyl-dodecylamine (DDAO) on catabolic activity of the Bacillus genus bacteria involved in the process of denitrificantion. The laboratory tests have proved that the presence of DDAO in concentrations greater than 75 ppm extends the time of nitrate (V) reduction, while the concentration of the intermediate-formed nitrates (III) is inhibited at the level of 80 mg/dm³. Further increase in DDAO concentration, to 10000 ppm, extends the nitrate (V) reduction time to about 115 hours, while the concentration of nitrates (III), formed in the second stage of the reaction, does nor change and remains at a level of 80-100 mg N-NO₂-/dm³. The process of denitrification is accompanied by a decrease in COD (biodegradation), whose concentration after completion of the process is 54% in the samples containing 75 ppm DDAO to 31% in the samples containing 10,000 ppm DDAO. Simultaneously, the concentration of proteins decreases from 70% to 30% in the medium containing DDAO in the concentration of 10,000 ppm.