Completely　autotrophic　nitrogen　removal　over　nitrite　(CANON)　has　been　regarded　as　an　efficient　and　economical　process　for　nitrogen　removal　from　wastewater.　The　distribution　and　genetic　diversity　of　the　functional　microorganisms　in　five　lab-scale　CANON　reactors　have　been　investigated　by　using　some　molecular　biology　methods.　Nitrosomonas-like　aerobic　ammonium　oxidizing　bacteria　(AerAOB)　and　Candidatus　Brocadia-related　anaerobic　ammonium　oxidizing　bacteria　(AnAOB)　were　detected　as　predominant　functional　microbes　in　the　five　reactors　while　Nitrobacter-like　nitrite　oxidizing　bacteria　(NOB)　existed　only　in　the　systems　operated　at　ambient　temperature.　Communities　of　AerAOB　and　AerAOB　were　almost　similar　among　the　five　reactors　while　the　distribution　of　the　functional　microbes　was　either　scattered　or　densely　packed.　Meanwhile,　this　study　has　demonstrated　the　feasibility　of　starting　up　CANON　by　inoculating　conventional　activated　sludge　in　low　ammonium　content　at　ambient　temperature.　(C)　2012　Elsevier　Ltd.　All　rights　reserved.