A conventional activated sludge system was used to treat the ammonium-bearing inorganic wastewater for 260 days under decreased HRTs. When treating 500 mg x L(-1) NH4+ -N wastewater, over 98% ammonia removal could be obtained at HRT > or =20 h. Sharp washout of biomass occurred when HRT was further decreased, resulting in the accumulation of ammonium and nitrite. Changes in the community structure were monitored by microbial quinone and conventional analytical methods. Quinone profiles indicated that ammonia-oxidizing bacteria (AOB) in beta-Protevbacteria and Nitrobacteria in alpha-Proteobacteria were the predominant species to oxidize ammonia and nitrite, respectively. The number of AOB decreased with the reduction of biomass in bioreactor, while that of nitrite-oxidizing bacteria (NOB) changed little and fluctuated at 10(5) CFU x L(-1)