Triclosan　(TCS)　is　commonly　found　in　wastewater　treatment　plants,　which　often　affects　biological　treatment　processes.　The　responses　of　nitrification,　antibiotic　resistome　and　microbial　community　under　different　TCS　concentrations　in　activated　sludge　system　were　evaluated　in　this　study.　The　experiment　was　conducted　in　a　sequencing　batch　reactor　(SBR)　for　240　days.　Quantitative　PCR　results　demonstrated　that　the　abundance　of　ammonium　oxidizing　bacteria　could　be　temporarily　inhibited　by　1　mg/L　TCS　and　then　gradually　recovered.　And　the　abundances　of　nitrite　oxidizing　bacteria　(NOB)　under　2.5　and　4　mg/L　TCS　were　three　orders　of　magnitude　lower　than　that　of　seed　sludge,　which　accounted　for　partial　nitrification.　When　the　addition　of　TCS　was　stopped,　the　abundance　of　NOB　increased.　The　mass　balance　experiments　of　TCS　demonstrated　that　the　primary　removal　pathway　of　TCS　changed　from　adsorption　to　biodegradation　as　TCS　was　continuously　added　into　the　SBR　system.　Moreover,　TCS　increased　the　abundance　of　mexB,　indicating　the　efflux　pump　might　be　the　main　TCS-resistance　mechanism.　As　a　response　to　TCS,　bacteria　could　secrete　more　protein　(PN)　than　polysaccharide.　Three-dimensional　excitation-emission　matrix　revealed　that　tryptophan　PN-like　substances　might　be　the　main　component　in　PN　to　resist　TCS.　High-throughput　sequencing　found　that　the　relative　abundances　of　Paracoccus,　Pseudoxanthomonas　and　Thauera　increased,　which　could　secrete　extracellular　polymeric　substances　(EPS).　And　Sphingopyxis　might　be　the　main　TCS-degrading　bacteria.　Overall,　TCS　could　cause　partial　nitrification　and　increase　the　relative　abundances　of　EPS-secreting　bacteria　and　TCS-degrading　bacteria.　(C)　2020　Published　by　Elsevier　B.V.　on　behalf　of　The　Research　Centre　for　Eco-Environmental　Sciences,　Chinese　Academy　of　Sciences.