It　is　generally　accepted　that　a　low　dissolved　oxygen　(DO)　concentration　is　more　beneficial　for　achieving　partial　nitrification　than　high-DO.　In　this　study,　partial　nitrification　was　not　established　under　low-DO　conditions　in　an　intermittent　aeration　reactor　for　treating　domestic　wastewater.　During　the　operational　period　of　low-DO　conditions　(DO:　0.3　+/-　0.14　mg/L),　stable　complete　nitrification　was　observed.　The　abundance　of　Nitrospira-like　bacteria,　which　were　the　major　nitrite-oxidizing　bacteria,　increased　from　1.03　x　10(6)　to　2.64　x　10(6)　cells/mL.　At　the　end　of　the　low-DO　period,　the　batch　tests　showed　that　high-DO　concentration　(1.5,　2.0　mg/L)　could　inhibit　nitrite　oxidation,　and　enhance　ammonia　oxidation.　After　switching　to　the　high-DO　period　(1.8　x　0.32　mg/L),　partial　nitrification　was　gradually　achieved.　Nitrospira　decreased　from　2.64　x　10(6)　to　8.85　x　10(5)　cells/mL.　It　was　found　that　suddenly　switching　to　a　high-DO　condition　could　inhibit　the　activity　and　abundance　of　Nitrospira-like　bacteria,　resulting　in　partial　nitrification.　(C)　2016　The　Research　Center　for　Eco-Environmental　Sciences,　Chinese　Academy　of　Sciences.　Published　by　Elsevier　B.V.