Selective nitrification was carried out to accumulate nitrite from high strength ammonia wastewater in an autotrophic nitrifying biofilm reactor. Nitrification efficiencies and nitrite accumulation characteristics were investigated at various operating conditions such as ammonium load, oxygen supply and free ammonia concentration. The biofilm. reactor showed very stable nitrification efficiencies of more than 90% at up to 2 kg NH4-N m(-3) d(-1) and the nitrite content was maintained at around 95%. Inhibition by free ammonia on nitrite oxidizers seems to be the major factor for nitrite accumulation. Batch kinetic analyses of ammonium and nitrite oxidation showed that nitrite oxidation activity was selectively inhibited in the presence of free ammonia. However, the activity recovered quickly as the free ammonia concentration decreased below the threshold inhibition concentration. Examination of specific ammonia and nitrite oxidation activities and the most probable number indicated that the number of nitrite-oxidizing microorganisms in the nitrite-accumulating system was less than that in the normal nitrification system due to long-term free ammonia inhibition of the nitrite oxidizers. The reduced population of nitrite oxidizers in the biofilm system was also responsible for the accumulation of nitrite in the biofilm reactor. (C) 2003 Society of Chemical Industry.