Community　structures　and　population　dynamics　of　nitrifying　bacteria　determine　biological　nitrogen　removal　from　municipal　wastewater.　The　population　structures　and　dynamics　of　ammonia-oxidizing　bacteria　(AOB)　and　nitrite-oxidizing　bacteria　(NOB)　in　four　full-scale　wastewater　treatment　plants　(WWTPs)　were　investigated　in　this　study.　Quantitative　real-time　PCR　results　showed　that　the　abundance　of　AOB　was　in　a　range　of　8.56×106~4.46×107　cells/gMLSS,　while　NOB　was　varying　in　3.37×108~1.53×109　cells/gMLSS.　In　each　process　Nitrospira　was　the　dominant　species　of　NOB.　Nitrospira　abundance　was　obviously　higher　than　Nitrobacter,　accounting　for　88%　of　total　NOB.　In　the　A2O　process　the　abundances　of　AOB　and　Nitrospira　in　winter　were　less　than　those　in　summer,　leading　to　decline　of　biological　nitrogen　removal.　The　phylogenetic　analysis　of　AOB　amoA　genes　indicated　that　all　the　sequences　were　affiliated　with　genera　Nitrosomonas,　among　which　Nitrosomonas　oligotropha　cluster　was　the　dominant　species,　accounting　for　60%　of　the　clone　libraries.　The　pre-dominant　AOB　were　Nitrosomonas-like　cluster　and　Nitrosomonas　europaea　cluster,　accounting　for　29.6%　and　9.1%　of　the　clone　libraries,　respectively.　N.　europaea　cluster　was　only　found　in　A2O　process,　and　reached　44.7%　of　total　AOB　in　summer　sample,　which　was　a　main　reason　causing　high　nitrite　accumulation　during　summer　operation　of　A2O　process.　The　outcomes　verified　that　the　dominant　AOB　and　NOB　in　WWTPs　was　Nitrosomonas　and　Nitrospira,　respectively.　Nitrifying　bacteria　accounted　for　1%~7%　of　total　bacteria.　The　abundances,　relative　distributions　and　community　structures　of　nitrifying　bacteria　significantly　influence　the　performance　of　biological　nitrogen　removal.　©,　2015,　Chinese　Society　for　Environmental　Sciences.　All　right　reserved.