A novel strain AL-6, which was identified as Acinetobacter baumannii, presented an efficient ability to remove ammonium. Nitrogen balance indicates that 55.8% of the initial NH4+-N was ultimately converted to N-2 through heterotrophic nitrification and aerobic denitrification, while 30.6% was contributed to assimilation. More interestingly, efficient ammonium removal could be achieved in the presence of Cr(VI) of 0-10 mg L-1 by strain AL-6. Meanwhile, Cr(VI) reduction was observed. The Cr(VI) was mainly reduced to less toxic Cr(III) by strain AL-6 in the culture, while a small part of Cr(VI) might be accumulated in bacterial cells in the formation of Cr(III). The optimal conditions for ammonium removal as well as Cr(VI) reduction was pH of 7 and C/N ratio of 10-15. This study provided a potential prospect for the treatment of Cr(VI)-containing ammonium wastewater. (C) 2020, The Society for Biotechnology, Japan. All rights reserved.