Shortcut　nitrification-denitrification　and　population　dynamics　of　ammonia　oxidizing　bacteria　(AOB)　and　nitrite　oxidizing　bacteria　(NOB)　were　investigated　in　a　continuous　flow　MUCT　reactor　treating　real　domestic　wastewater.　Shortcut　nitrification-denitrification　was　achieved　by　controlling　low　dissolved　oxygen　(DO)　concentration　of　0.5　mgL-1　and　short　hydraulic　retention　time　(HRT)　of　6　h.　Nitrite　accumulation　ratios　reached　above　90%.　TN　removal　in　the　phase　of　shortcut　nitrification-denitrification　was　up　to　90%,　far　higher　than　complete　nitrification　with　74%　of　TN　removal.　The　results　of　quantitative　real　time　PCR　(QPCR)　indicated　that　during　complete　nitrification-denitrification　AOB　abundance　exhibited　a　decline　tendency,　decreasing　from　1.69109　cells(g　dried　sludge)-1　to　3.76107　cells(g　dried　sludge)-1,　and　NOB　abundance　maintained　at　108　cells(g　dried　sludge)-1.　During　shortcut　nitrification-denitrification,　the　abundance　of　AOB　slightly　increased　from　3.17106　cells(g　dried　sludge)-1　to　1.32107　cells(g　dried　sludge)-1　accompanied　with　a　slight　increase　of　ratio　of　AOB　to　total　bacteria.　The　abundance　of　NOB　fluctuated　in　a　range　of　5.9107-1.78108　cells(g　dried　sludge)-1.　The　ratio　of　NOB　to　total　bacteria　dropped　from　1.44%　to　0.47%.　Therefore,　the　abundance　decrease　and　bioactivities　inhibition　of　NOB　were　the　important　factors　to　achieve　shortcut　nitrification-denitrification　in　MUCT　process　treating　real　domestic　wastewater.　During　shortcut　nitrification-denitrification　due　to　low　DO　concentration　and　short　HRT,　AOB　abundance　and　their　relative　distribution　did　not　increase,　even　descended.　But　that　did　not　influence　the　removal　of　ammonia　and　total　nitrogen.　©　All　Right　Reserved.