＂Candidatus　Accumulibacter＂　are　the　dominant　polyphosphate-accumulating　organism　(PAOs)　to　perform　enhanced　biological　phosphate　removal　(EBPR)　from　wastewater.　Free　nitrous　acid　(FNA)　as　inhibitor　is　an　intermediate　of　nitrification　and　denitrification　commonly　present　in　wastewater　treatment　plants　(WWTPs)　and　lab-scale　reactors.　Impact　of　FNA　on　EBPR　and　community　structures　of　Accumulibacter　in　domestic　wastewater　treatment　was　investigated.　FNA　complete　inhibit　concentration　on　aerobic　P　uptake　at　pH　6.5　was　6.85　x　10(-3)　mg　HNO2-N/L,　higher　than　that　at　pH　7.0.　Under　complete　inhibition,　poly-p-hydroxyalkanoates　degradation　was　used　for　nitrite　reduction　and　detoxication　rather　than　P-uptake.　The　P　fraction　existed　in　the　forms　of　complex-P,　interstitial-P,　chemical　participated-P　and　ortho-P　at　the　end　of　aerobic　phase,　and　complex-P　proportion　declined　in　comparison　to　non　inhibition.　Dominant　Accumulibacter　Glade　was　BC,　accounting　for　70%　of　total　Accumulibacter.　Decline　of　IIF　proportion　indicated　its　sensitivity　to　FNA.　Increase　of　HD　percentage　implied　its　strong　tolerance　to　FNA.　Under　inhibition,　Shannon　index　decreased　from　3.45　of　seed　sludge　to　1.92　at　pH　7.0,　indicating　decline　of　biodiversity.　Therefore,　FNA　inhibition　weakened　EBPR　performance,　and　remarkably　changed　Glade-level　community　structures　of　Accumulibacter.　(C)　2016　Elsevier　B.V.　All　rights　reserved.