The heterotrophic nitrification and aerobic denitrification capabilities of Bacillus sp. LY were investigated under the aerobic condition. The results indicate that Bacillus sp. LY is not only a heterotrophic nitrifier, but also an aerobic denitrifier. Experiments were carried out in an attempt to determine and quantify the contribution of heterotrophic nitrification and aerobic denitrification to total N removal. By taking the nitrogen balance under the culture condition of 41.1 mg/L of initial NH₄ ⁺‐N at a C/N ratio of 15 in 96 h, 8.0% of the initial NH₄ ⁺‐N still remained in the medium in the forms of hydroxylamine, nitrite, nitrate and organic N; 40.5% of NH₄ ⁺‐N was converted to biomass and 45.9% of NH₄ ⁺‐N was estimated to be finally removed in the formation of N₂. This conversion of ammonium to N₂ with the intermediate formation of N₂O under the aerobic condition was confirmed by gas chromatography. Single step nitrogen removal by simultaneous heterotrophic nitrification and aerobic denitrification has great potential in wastewater treatment.