In　this　study,　a　lab-scale　partial　nitrifying　sequencing　batch　reactor　(SBR)　was　developed　to　investigate　partial　nitrification　at　ambient　temperature　(16-22　A　degrees　C).　Techniques　of　denaturing　gradient　gel　electrophoresis　(DGGE),　cloning,　and　fluorescence　in　situ　hybridization　(FISH)　were　utilized　simultaneously　to　study　microbial　population　dynamics.　Partial　nitrification　was　effectively　achieved　in　response　to　shifts　of　influent　ammonium　concentrations.　DGGE　results　showed　that　higher　ammonia　concentration　referred　to　lower　ammonia-oxidizing　bacteria　(AOB)　diversity　in　the　SBR.　Phylogenetic　analysis　revealed　that　all　the　predominant　AOB　was　affiliated　with　Nitrosomonas　genus.　FISH　analysis　illustrated　AOB　was　the　predominant　nitrifying　bacteria　of　microbial　compositions　when　SBR　achieved　partial　nitrification　(PN)　at　ambient　temperature.