Inactivation　and　adaptation　of　ammonia-oxidizing　bacteria　(AOB)　and　nitrite-oxidizing　bacteria　(NOB)　to　free　nitrous　acid　(FNA)　was　investigated.　Batch　test　results　showed　that　AOB　and　NOB　were　inactivated　when　treated　with　FNA.　After　an　85-day　operating　period,　AOB　in　a　continuous　pre-denitrification　reactor　did　not　adapt　to　the　FNA　that　was　applied　to　treat　some　of　the　return　activated　sludge.　In　contrast,　NOB　did　adapt　to　FNA.　NOB　activity　in　the　seed　sludge　was　only　11%　of　the　original　activity　after　FNA　batch　treatment,　at　0.75　mg　HNO2-N/L.　NOB　activity　in　the　pre-denitrification　reactor　was　not　affected　after　being　exposed　to　this　FNA　level.　Nitrosomonas　was　the　dominant　AOB　before　and　after　long-term　FNA　treatment.　However,　dominant　NOB　changed　from　Nitrospira　to　Candidatus　Nitrotoga,　a　novel　NOB　genus,　after　long-term　FNA　treatment.　This　adaptation　of　NOB　to　FNA　may　be　due　to　the　shift　in　NOB　population　makeup.