This　study　investigated　the　diversity　and　abundance　of　AOA　and　AOB　amoA　genes　in　eight　full-scale　wastewater　treatment　plants　(WWTPs).　Although　the　process　principles　and　system　operations　of　the　eight　WWTPs　were　different,　quantitative　real-time　PCR　measurements　showed　that　AOB　amoA　genes　outnumbered　AOA　amoA　genes　with　the　ratio　varying　from　2.56　to　2.41　x　10(3),　and　ammonia　may　be　partially　oxidized　by　AOA.　Phylogenetic　analyses　based　on　cloning　and　sequencing　showed　that　Nitrososphaera　cluster　was　the　most　dominant　AOA　species　and　might　be　distributed　worldwide,　and　Nitrosopumilis　cluster　was　few.　Statistical　analysis　indicated　that　there　might　be　versatile　AOA　ecotypes　and　some　AOA　might　be　not　obligate　autotrophic.　The　Nitrosomonas　europaea　cluster　and　Nitrosomonas　oligotropha　cluster　were　the　two　most　dominant　AOB　species,　and　AOB　species　showed　higher　diversity　than　AOA　species.　(C)　2013　Elsevier　Ltd.　All　rights　reserved.