Phenacetin　(PNCT),　a　common　antipyretic　and　analgesic　drug,　is　often　used　to　treat　fever　and　headache.　However,　the　effect　of　PNCT　on　nitrifiers　in　wastewater　treatment　processes　remains　unclear.　The　practicability　of　attaining　partial　nitrification　(PN)　through　inhibitor-PNCT　was　investigated　in　this　study.　The　optimal　treatment　conditions　of　soaking　once　for　18　h　with　2.50　x　10(-3)　g　PNCT/(g　MLSS)　were　applied　to　the　PN　stability　experiment.　The　results　showed　that　ammonia　oxidation　activity　recovered　quickly　after　3　cycles　of　operation,　while　nitrite　oxidation　activity　was　suppressed　steadily.　In　addition,　average　ammonium　removal　efficiency　and　nitrite　accumulation　ratio　during　138　cycles　could　reach　94.94%　and　85.38%,　respectively.　Complimentary　DNA　high-throughput　sequencing　and　oligotyping　analysis　showed　that　the　activity　of　Nitrosomonas　would　gradually　surpass　Nitrospira　after　PNCT　treatment　only　once.　The　decrease　of　Nitrospira　activity　was　accompanied　by　the　simplification　of　oligotypes　after　PNCT　treatment,　while　Nitrosomonas　could　adapt　to　PNCT　stress　by　reducing　the　differences　between　oligotypes.　Metagenomics　revealed　that　the　decrease　in　the　number　of　NXR　in　the　nitrogen　metabolism　pathways　was　the　key　reason　for　achieving　PN.　The　potential　mechanisms　might　be　that　the　dominant　nitrite-oxidizing　bacteria　and　complete　ammonia　oxidizers　were　bio-killed　by　PNCT.　(C)　2021　Elsevier　B.V.　All　rights　reserved.