The　abundance　and　diversity　of　anammox　bacteria　was　investigated　in　two　pilot-scale　integrated　fixed-film　activated　sludge　(IFAS)　reactors　treating　high　ammonium　wastewater.　Reactor　A　was　inoculated　with　nitrifying　sludge,　while　Reactor　B　was　inoculated　with　suspended　anammox　sludge　with　the　dominant　anammox　bacteria　of　Candidatus　＇Kuenenia＇.　After　180　days＇　operation,　the　predominate　anammox　bacteria　was　Candidatus　＇Brocadia＇　(65%)　in　the　biofilm,　while　Candidatus　＇Kuenenia＇　(86%)　outcompeted　with　other　anammox　bacteria　in　suspended　sludge　in　Reactor　A.　Candidatus　＇Kuenenia＇　were　dominated　in　suspended　sludge　through　the　entire　experiment　in　Reactor　B.　In　contrast,　the　predominated　species　shifted　from　Candidatus　＇Kuenenia＇　(89%)　into　Candidatus　＇Brocadia＇　(66%)　in　the　biofilm　of　Reactor　B.　This　study　indicated　that　Candidatus　＇Brocadia＇　preferred　to　grow　in　the　biofilm,　while　Candidatus　＇Kuenenia＇　would　dominant　over　other　anammox　bacteria　in　the　suspended　sludge.　Further　studies　are　required　to　identify　the　internal　factors　affecting　the　distribution　of　anammox　bacteria.　(C)　2016　Elsevier　Ltd.　All　rights　reserved.