Ammonia-oxidizing　bacteria　(AOB)　and　archaea　(AOA)　are　two　distinct　ammonia-oxidizing　microorganisms　(AOMs)　responsible　for　nitrification　in　wastewater　treatment　plants　(WWTPs).　However,　their　relative　contributions　to　ammonia　oxidation　in　WWTPs　and　short-term　responses　to　titanium　dioxide　nanoparticles　(TiO2　NPs)　remain　unclear.　In　this　study,　DNA-based　stable　isotope　probing　(DNA-SIP),　quantitative　polymerase　chain　reaction　(qPCR),　PCR,　cloning　and　sequencing　were　applied　to　investigate　the　in　situ　activity　of　AOMs　in　a　full-scale　WWTP　and　to　evaluate　their　responses　to　TiO2　NPs.　An　environmentally　relevant　concentration　(1　mg　L-1)　and　a　higher　concentration　(50　mg　L-1)　of　TiO2　NPs　were　chosen　for　laboratory　microcosms.　The　laboratory　microcosms　revealed　that　TiO2　NPs　(1　or　50　mg　L-1)　caused　slight　or　no　short-term　inhibitions　on　nitrification　activity　and　nitrification　rate　of　activated　sludge.　The　successful　(Na2CO3)-C-13　assimilation　of　AOB　rather　than　AOA　in　original　sludge　provided　compelling　evidence　for　the　strong　contribution　of　AOB　to　the　in　situ　nitrification　of　the　WWTP.　The　main　AOB　in　original　sludge,　Nitrosomonas　oligotropha　cluster　and　Nitrosomonas　europaea　cluster,　were　the　dominant　active　AOB　in　the　WWTP.　However,　compared　with　the　original　sludge,　the　abundance　and　distribution　of　active　AOB　changed　under　1　and　50　mg　L-1　of　TiO2　NPs.　In　particular,　AOA　within　Nitrososphaera　cluster　were　the　active　AOMs　under　50　mg　L-1　TiO2　NPs.　These　results　suggested　that　TiO2　NPs　had　a　potential　impact　on　the　abundance　and　composition　of　active　AOMs　in　the　WWTP.　This　study　further　provided　direct　evidence　of　the　autotrophic　growth　of　AOB　within　N.　oligotropha　cluster　and　N.　europaea　cluster,　and　AOA　within　the　general　Nitrososphaera　cluster.　Overall,　AOB　played　an　important　role　in　the　in　situ　nitrification　of　the　full-scale　WWTP　investigated,　while　AOA　showed　a　strong　contribution　to　the　active　nitrification　under　higher　concentration　of　TiO2　NPs.