The　abundance　and　diversity　of　amoA　genes　of　ammonia-oxidizing　archaea　(AOA)　and　bacteria　(AOB)　were　investigated　in　ten　wastewater　treatment　systems　(WTSs)　by　polymerase　chain　reaction　(PCR),　cloning,　sequencing,　and　quantitative　real-time　PCR　(qPCR).　The　ten　WTSs　included　four　full-scale　municipal　WTSs,　three　full-scale　industrial　WTSs,　and　three　lab-scale　WTSs.　AOB　were　present　in　all　the　WTSs,　whereas　AOA　were　detected　in　nine　WTSs.　QPCR　data　showed　that　AOB　amoA　genes　(4.625　x　10(4)-9.99　x　10(9)　copies　g(-1)　sludge)　outnumbered　AOA　amoA　genes　(＜　limit　of　detection-1.90　x　10(7)　copies　g(-1)　sludge)　in　each　WTS,　indicating　that　AOB　may　play　an　important　role　than　AOA　in　ammonia　oxidization　in　WTSs.　Interestingly,　it　was　found　that　AOA　and　AOB　coexisted　with　anaerobic　ammonia　oxidation　(anammox)　bacteria　in　three　anammox　WTSs　with　relatively　higher　abundance.　In　a　full-scale　industrial　WTS　where　effluent　ammonia　was　higher　than　influent　ammonia,　both　AOA　and　AOB　showed　higher　abundance.　The　phylogenetic　analysis　of　AOB　amoA　genes　showed　that　genera　Nitrosomonas　was　the　most　dominant　species　in　the　ten　WTSs;　Nitrosomonas　europaea　cluster　was　the　dominant　major　cluster,　followed　by　Nitrosomonas-like　cluster　and　Nitrosomonas　oligotropha　cluster;　and　AOB　species　showed　higher　diversity　than　AOA　species.　AOA　were　found　to　be　affiliated　with　two　major　clusters:　Nitrososphaera　cluster　and　Nitrosopumilus　cluster.　Nitrososphaera　cluster　was　the　most　dominant　species　in　different　samples　and　distributed　worldwide.