BACKGROUND: The co-existence of NH3-N and Cr(VI) in natural water is a common and growing problem. The electron competition ability of Cr(VI) with NO3-N/ NO2-N enabled the reduction of Cr(VI) complex during the process of simultaneous nitrification/denitrification (SND). The current study focused on exploring the interaction between Cr(VI) reduction and SND by Acinetobacter sp. WB-1. RESULTS: Different Cr(VI) concentrations had no effect on NH3-N removal, while increased Cr(VI) concentration (1-30mgL(-1)) could affect the rates of reduction of NO3-N/NO2-N. However, NH3-N at concentrations ranging from 100 to 400mgL(-1) had no influence on Cr(VI) reduction. The location/contribution of cellular components demonstrated cytoplasm and filtered spent medium had the ability to reduce Cr(VI) to Cr(III). Cell debris could not reduce Cr(VI) but could accumulate Cr(III). CONCLUSION: The denitrification process was more sensitive to the presence of Cr(VI) than the nitrification process. The inhibition of NO3-N reduction was due to the toxicity and electron competition ability of Cr(VI). The inhibitory effect on NO2-N reduction was mainly dependent on the toxicity of Cr(VI) to NO2-N reductase. Cr(VI) removal by strain WB-1 was suggested to be realized through surface immobilization along with intracellular and extracellular reduction. (c) 2016 Society of Chemical Industry