Paracoccus versutus LYM was characterized with the ability of simultaneous nitrification and denitrification (SND) in presence of NH4+-N and NO3--N (140 mg/L-N each). However, there was approximate 95.80 mg/L NO2--N left during SND process. Fe2+ was added in the SND medium and it benefited bacterial growth and further converted NO2--N a little. Besides, remaining NO2--N could be completely reduced by adding carbon donors in presence of NH4+-N and NO3--N, whose converting efficiencies also reached 97.09% and 96.34%, respectively. And N-2 stayed as the main product of SND process. While NO2--N and NH4+-N were present simultaneously, strain LYM could also remove nitrogen compounds thoroughly with sufficient carbon source. But there was no NO2--N reduced as sole nitrogen source in the medium even if C/N ratio reached as high as 30. (C) 2014 Elsevier Ltd. All rights reserved.