The　effective　inhibition　of　nitrite　oxidizing　bacteria　(NOB)　is　the　key　to　realizing　satisfactory　nitrite　accumulation　and　achieving　effective　nitritation.　In　order　to　explore　the　selective　effect　of　hydroxylamine　(NH2OH)　on　ammonia　oxidizing　bacteria　(AOB)　and　NOB,　a　sequencing　batch　reactor　(SBR)　with　the　operation　mode　of　anaerobic/aerobic/anoxia　(A/O/A)　was　used　to　observe　the　start-up　of　nitritation　at　different　concentrations　and　frequencies　of　NH2OH.　The　results　showed　that　when　5　mg•L-1　of　NH2OH　was　added　once　every　2　cycles,　the　nitrite　accumulation　rate　(NAR)　increased　from　0.1%　to　57.4%　in　6　days,　and　was　maintained　at　(62.0±4.6)%　until　the　end　of　the　trials.　In　the　typical　cycle　on　day　6,　the　NN4+-N　dropped　from　26.05　mg•L-1　to　8.06　mg•L-1,　thus　producing　9.02　mg•L-1　of　NO2--N　and　6.70　mg•L-1　of　NO3--N.　Meanwhile,　the　ratio　of　the　maximum　activity　of　AOB　(rAOB)　to　NOB　(rNOB)　increased　from　1.05　on　day　1　to　4.22　on　day　9.　Moreover,　qPCR　results　indicated　that　the　abundance　of　AOB　and　NOB　decreased　to　30.2%　and　19.1%,　respectively,　on　day　9　in　comparison　to　the　original　sample.　The　results　indicate　that　the　selective　effect　of　AOB　and　NOB　based　on　NH2OH　is　expected　to　provide　a　feasible　application　for　the　rapid　start-up　nitritation　of　municipal　wastewater.　©　2020,　Science　Press.　All　right　reserved.