Benzalkyl　dimethylammonium　compounds　(BACs)　are　generally　applied　as　surfactants　and　disinfectants.　In　this　study,　the　nitrification　systems　were　exposed　to　different　alkyl　chain　lengths　(C12-C16)　and　different　levels　of　BACs　(0-5　mg/L),　respectively,　totally　120　days　and　to　explore　the　chronic　effect　of　BACs　on　resistance　genes　(RGs).　RGs　were　classified　into　four　fractions　based　on　activated　sludge　properties.　Ammonia　oxidation　per-formance　were　not　significantly　affected　by　BACs,　whereas　BACs　increased　the　absolute　abundance　of　most　intracellular　RGs　in　sludge　(si-RGs).　Under　the　exposure　of　BACs,　extracellular　RGs　in　water　(we-RGs)　showed　a　decrease　trend　and　si-RGs　tended　to　be　converted　to　we-RGs.　Tightly　bound-Tyrosine　side　chain　was　significantly　correlated　with　most　we-RGs,　and　we-intI1　might　contribute　to　the　propagation　of　RGs.　Therefore,　the　risk　of　transmission　of　different　fractions　of　RGs　in　the　nitrification　system　under　the　stress　of　BACs　should　be　taken　seriously.