Impact　of　nitrite　on　aerobic　phosphorus　(P)　uptake　of　poly-phosphate　accumulating　organisms　(PAOs)　in　three　different　enhanced　biological　phosphorus　removal　(EBPR)　systems　was　investigated,　i.e.,　the　enriched　PAOs　culture　fed　with　synthetic　wastewater,　the　two　lab-scale　sequencing　batch　reactors　(SBRs)　treating　domestic　wastewater　for　nutrient　removal　through　nitrite-pathway　nitritation　and　nitrate-pathway　nitrification,　respectively.　Fluorescence　in　situ　hybridization　results　showed　that　PAOs　in　the　three　sludges　accounted　for　72,　7.6　and　6.5　%　of　bacteria,　respectively.　In　the　enriched　PAOs　culture,　at　free　nitrous　acid　(FNA)　concentration　of　0.47　x　10(-3)　mg　HNO2-N/L,　aerobic　P-uptake　and　oxidation　of　intercellular　poly-beta-hydroxyalkanoates　were　both　inhibited.　Denitrifying　phosphorus　removal　under　the　aerobic　conditions　was　observed,　indicating　the　existence　of　PAOs　using　nitrite　as　electron　acceptor　in　this　culture.　When　the　FNA　concentration　reached　2.25　x　10(-3)　mg　HNO2-N/L,　denitrifying　phosphorus　removal　was　also　inhibited.　And　the　inhibition　ceased　once　nitrite　was　exhausted.　Corresponding　to　both　SBRs　treating　domestic　wastewater　with　nitritation　and　nitrification　pathway,　nitrite　inhibition　on　aerobic　P-uptake　by　PAOs　did　not　occur　even　though　FNA　concentration　reached　3　x　10(-3)　and　2.13　x　10(-3)　mg　HNO2-N/L,　respectively.　Therefore,　PAOs　taken　from　different　EBPR　activated　sludges　had　different　tolerance　to　nitrite.