Simultaneous nitrification and denitrification using a mixed methanotrophic culture was investigated. When both NO3--N (108 mg l(-1)) and NH3-N (59 mg l(-1)) were added into batch reactors, nitrate removal was complete within 10 h at the rate of 47 mg NO3--N g VSS-1 day(-1) when dissolved oxygen (DO) concentration was maintained at 2 mg DO l(-1). Ammonia removal started simultaneously with nitrate removal at a slower rate of 14 NH3-N g VSS-1 day(-1). No significant accumulation of nitrite or nitrate during ammonia utilization suggested the occurrence of simultaneous nitrification and denitrification.