Two　enhanced　biological　phosphorus　removal　(EBPR)　sludges　were　used　to　investigate　the　effect　of　nitrite　on　anaerobic　metabolism　of　phosphate　accumulating　organisms　(PAOs).　It　is　found　that　tolerance　of　PAOs　to　nitrite　in　two　sludges　were　different.　In　the　sludge　with　enriched　PAOs　cultures　treating　synthetic　wastewater,　when　nitrite　is　above　10　mg/L,　it　inhibits　VFA-uptake　of　PAOs,　leading　to　decrease　of　PHA　synthesis　and　increase　of　phosphate　release.　In　the　sludge　with　nitrogen　and　phosphorus　removal　by　shortcut　nitrification　treating　real　domestic　wastewater,　anaerobic　metabolism　of　PAOs　is　not　inhibited　even　when　nitrite　is　30　mg/L;　however,　the　presence　of　nitrite　in　anaerobic　period　causes　denitrifying　bacteria　compete　for　limited　carbon　sources　with　PAOs,　resulting　in　decrease　of　PHA　synthesis　and　phosphate　release.　For　the　enriched　PAOs　cultures,　the　ratio　of　P/VFA　obviously　increases　when　nitrite　is　added　in　the　reactor,　indicating　increased　energy　requirement　for　PAOs　to　take　up　VFA.　The　quantification　results　for　PAOs　in　two　sludges　using　fluorescence　in-situ　hybridization　(FISH)　show　that　the　PAOs　level　is　55%　in　the　enriched　PAOs　cultures,　and　7.6%　in　the　sludge　with　nitrogen　and　phosphorus　removal　by　shortcut　nitrification.