Population　dynamic　of　nitrifying　bacteria　was　investigated　for　nitrogen　removal　from　municipal　waste-water.　Nitritation　was　established　with　nitrite　accumulation　ratios　above　85%.　Quantitative　PCR　indicated　that　Nitrospira　was　dominant　nitrite　oxidizing　bacteria　(NOB)　and　Nitrobacter　was　few.　During　nitritation　achieving,　Nitrobacter　was　firstly　eliminated,　along　with　inhibition　of　Nitrospira　bioactivities,　then　Nitrospira　percentage　declined　and　was　finally　washed　out.　Nitritation　establishment　depended　on　inhibiting　and　eliminating　of　NOB　rather　than　ammonia　oxidizing　bacteria　(AOB)　enriching.　This　is　the　first　study　where　population　dynamics　of　Nitrobacter　and　Nitrospira　were　investigated　to　reveal　mechanism　of　nitritation　in　a　continuous-flow　process.　Phylogenetic　analysis　of　AOB　indicated　that　Nitrosomonas-like　cluster　and　Nitrosomonas　oligotropha　were　dominant　AOB,　accounting　for　81.6%　of　amoA　gene　clone　library.　Community　structure　of　AOB　was　similar　to　that　of　complete　nitrification　system　with　long　hydraulic　retention　time,　but　different　from　that　of　nitritation　reactor　with　low　DO　concentration.　(C)　2014　The　Authors.　Published　by　Elsevier　Ltd.