Alcaligenes faecalis no. 4 has heterotrophic nitrification and aerobic denitrification abilities. By taking the nitrogen balance under different culture conditions, 40-50% of removed NH4+-N was denitrified and about one-half of removed NH4+-N was converted to intracellular nitrogen. The maximum ammonium removal rate of no. 4 (28.9 mg-N/l/h) and its denitrification rate at high-strength NH4+-N of about 1200 ppm in aerated batch experiments at a C/N ratio of 10 were 5-40 times higher than those of other bacteria with the same ability. Only a few percent of the removed ammonium was converted to nitrite, and the main denitrification process was speculated to be via hydroxylamine which was produced by ammonium oxidation.