Simultaneous nitrification and denitrification in a Pasveer oxidation ditch was studied. The purpose was to evaluate the performances of both nitrification and dentrification in oxidation ditches, and to pursue some possible approaches to enhance nitrogen removal. Almost complete nitrification was observed under normal conditions, but when DO<1.0 mg/l andior T<4 degrees C, nitrification was profoundly affected. Also because of low temperature, transient nitrite accumulation (up to 6.7 mg N/l) appeared on two occasions. It was concluded that for nitrification there was a strong capacity to absorb shock nitrogen loading. However denitrification was unstable and poor; the average efficiency of nitrogen removal was only 45% during a study period of almost 10 months. The bottleneck to denitrification was attributed to the lack of stable anoxic zones and carbon compounds available to denitrifiers. Several technical approaches are strongly proposed to improve denitrification. (C) 1997 Elsevier Science Ltd.