To　achieve　nitritation　from　complete-nitrification　seed　sludge　at　room　temperature　of　19　+/-　1　degrees　C,　a　lab-scale　sequencing　batch　reactor　(SBR)　treating　domestic　wastewater　with　low　C/N　ratios　was　operated　to　investigate　the　control　and　optimization　of　nitrifying　communities.　Ammonia　oxidizing　bacteria　(AOB)　dominance　was　enhanced　through　the　combination　of　low　DO　concentrations　(＜1.0　mg/L)　and　preset　short-cycle　control　of　aeration　time.　Nitritation　Was　Successfully　established　with　NO2--N/NOx--N　over　95%.　To　avoid　the　adverse　impact　of　low　DO　concentrations　on　AOB　activities,　DO　concentrations　were　increased　to　1-2　mg/L.　At　the　normal　DO　levels　and　temperatures,　on-line　control　strategy　of　aerobic　durations　maintained　the　stability　of　nitritation　with　nitrite　accumulation　rate　over　95%　and　ammonia　removal　above　97%.　Fluorescence　in-situ　hybridization　(FISH)　analysis　presented　that　the　maximal　percentage　of　AOB　in　biomass　reached　10.9%　and　nitrite　oxidizing　bacteria　(NOB)　were　washed　out.　(C)　2009　Elsevier　Inc.　All　rights　reserved.