Dynamics　of　nitrification　activity,　ammonia-oxidizing　archaea　(AOA)　and　bacteria　(AOB)　abundance　and　active　ammonia　oxidizers　of　activated　sludge　were　explored　under　different　salinities.　Results　showed　that　specific　ammonium　oxidation　rates　were　significantly　negative　with　increasing　salinity.　The　responses　of　AOA　and　AOB　populations　to　salt　stress　were　distinct.　AOA　abundance　decreased　at　moderate　salinities　(2.5,　5　and　7　g　L-1)　and　increased　at　high　salinities　(10,　15,　20　and　30　g　L-1),　while　AOB　abundance　showed　opposite　tendency.　DNA-based　stable　isotope　probing　assays　indicated　AOA　exclusively　dominated　active　ammonia　oxidation　of　test　samples　under　different　salinities.　The　active　AOA　communities　retrieved　were　all　non-halophilic　and　regulated　by　salinities.　Candidatus　Nitrosocosmicus　exaquare　and　Ca.　Nitrosocosmicus　franklandus　were　the　predominantly　active　AOA　in　both　salt-free　and　salt-containing　microcosms,　while　C-13-labeled　Nitrososphaera　viennensis　and　Ca.　Nitrososphaera　gargensis　were　only　retrieved　from　the　microcosms　amended　with　0　and　30　g　L-1　salinity,　respectively.