In　situ　enrichment　of　anammox　bacteria　in　anoxic　biofilms　has　been　observed,　but　the　specific　conditions　for　anammox　competition　with　denitrification　for　nitrite　are　not　yet　fully　understood.　Therefore,　an　anoxic　sequencing　batch　biofilm　reactor　(SBBR)　was　used　to　investigate　nitrite　production　during　denitrification.　In　each　SBBR　cycle,　with　nearly　80%　of　nitrate　reduced　in　3　h,　over　9.7　mg/L　nitrite　was　gradually　accumulated　and　maintained　for　a　long　time,　despite　temperatures　gradually　decreasing　from　32　to　5　degrees　C.　The　long-term　existence　of　nitrite　was　due　to　the　low　biofilm　nitrite　reduction　rate　(1.2　mgN　gVSS(-1)　h(-1)),　which　was　about　10-fold　less　than　the　nitrate　reduction　rate.　Accordingly,　nitrite　reduction　via　denitrifiers　was　continuously　suppressed,　which　was　favorable　for　nitrite　reduction　through　the　anammox　pathway.　Indeed,　anammox　bacteria　were　successfully　enriched　here　(Candidatus　Brocadia,　0.1%).　This　study　confirms　the　potential　of　anoxic　biofilm　in　enriching　anammox　bacteria　and　provides　insight　into　understanding.