In　this　study,　the　performance　of　partial　nitrification　via　nitrite　and　microbial　community　structure　were　investigated　and　compared　in　two　sequencing　batch　reactors　(SBR)　with　different　dissolved　oxygen　(DO)　levels.　Both　reactors　achieved　stable　partial　nitrification　with　nitrite　accumulation　ratio　of　above　95%　by　using　real-time　aeration　duration　control.　Compared　with　high　DO　(above　3　mg/l　on　average)　SBR,　simultaneous　nitrification　and　denitrification　(SND)　via　nitrite　was　carried　out　in　low　DO　(0.4-0.8　mg/l)　SBR.　The　average　efficiencies　of　SND　in　high　DO　and　low　DO　reactor　were　7.7%　and　44.9%,　and　the　specific　SND　rates　were　0.20　and　0.83　mg　N/(mg　MLSS　h),　respectively.　Low　DO　did　not　produce　sludge　with　poorer　settling　properties　but　attained　lower　turbidities　of　the　effluent　than　high　DO.　Fluorescence　in　situ　hybridization　(FISH)　analysis　in　both　the　reactors　showed　that　ammonia-oxidizing　bacteria　(AOB)　were　the　dominant　nitrifying　bacteria　and　nitrite-oxidizing　bacteria　(NOB)　did　not　be　recovered　in　spite　of　exposing　nitrifying　sludge　to　high　DO.　The　morphology　of　the　sludge　from　both　two　reactors　according　to　scanning　electron　microscope　indicated　that　small　rod-shaped　and　spherical　clusters　were　dominant,　although　filamentous　bacteria　and　few　long　rod-shaped　coexisted　in　the　low　DO　reactor.　By　selecting　properly　DO　level　and　adopting　process　control　method　is　not　only　of　benefit　to　the　achievement　of　novel　biological　nitrogen　removal　technology,　but　also　favorable　to　sludge　population　optimization.　Crown　Copyright　(c)　2009　Published　by　Elsevier　Ltd.　All　rights　reserved.