Three　lab-scale　automatic　control　sequencing　batch　reactors　(SBR)　were　operated　by　using　acetate　as　the　single　carbon　source,　nitrite　was　dosed　to　SBR-1　and　SBR-2　under　anoxic　and　aerobic　conditions,　respectively,　and　SBR　3　was　operated　as　control　without　dosing.　The　results　show　that,　the　presence　of　nitrite　under　anoxic　and　aerobic　conditions　would　both　cause　filamentous　sludge　bulking.　A　more　serious　sludge　bulking　is　observed　when　the　nitrite　is　presented　under　the　anoxic　conditions.　Nitrite　could　stimulate　the　increase　of　carbohydrates　in　extracellular　polymeric　substances,　weaken　the　storage　capability　of　floc-formers　and　decrease　the　phosphorous　removal　efficiency,　thus　cause　the　deterioration　of　sludge　settleability.　The　dominant　filaments　are　both　Thiotrix　nivea　for　SBR-1　and　SBR-2　when　sludge　bulking　occurred.