The　influence　of　nitrogen　(N),　phosphate　(P)　and　N&P　deficiency　on　sludge　settleability　and　filamentous　growth　was　investigated　in　four　sequencing　batch　reactors.　The　results　showed　that　P　deficiency　in　influent　would　cause　decrease　of　polyphosphate-accumulating　organisms　(PAOs)　and　inhibit　the　storage　capability　of　floc-formers.　Moreover,　the　proportion　of　protein　in　extracellular　polymer　substances　(EPS)　decreased　and　sludge　volume　index　(SVI)　of　200-250　ml·g-1　was　observed.　N　deficiency　in　influent　had　less　impact　on　the　growth　of　PAOs　and　the　storage　capability　of　floc-formers.　Protein　in　EPS　was　significantly　improved,　and　SVI　of　less　than　100　ml·g-1　was　observed　under　this　condition.　The　short-term　deficiency　of　N&P　in　influent　would　cause　deterioration　of　sludge　settleability,　but　no　further　deterioration　was　observed　after　long-term　domestication.　No　excessive　growth　of　filaments　but　large　numbers　of　spherical　floc-formers　existed　in　the　N&P　deficiency　system,　resulting　in　SVI　of　around　120　ml·g-1.　The　dominant　filaments　for　N,　P　and　N&P　deficiency　were　N.　limicola　II,　Type　0092　and　N.　limicola　I,　respectively.　All　of　these　dominant　filaments　had　limited　effect　on　sludge　settleability.　©　All　Rights　Reserved.