A　strain　with　high　tolerance　to　high-strength　ammonia,　which　grew　and　metabolized　at　an　ammonia　concentration　of　2,000　mg/L,　was　isolated　from　activated　sludge.　The　strain　was　identified　as　Acinetobacter　sp.　by　morphological　and　phylogenetic　16S　rDNA　analyses.　Based　on　the　high　removal　efficiency　of　ammonia/nitrate　and　enzyme　assays,　the　strain　was　confirmed　to　exhibit　heterotrophic　nitrification　and　aerobic　denitrification　abilities　with　limited　accumulation　of　intermediates.　The　inhibitory　effect　of　free　ammonia　(FA)　on　Acinetobacter　sp.　was　evaluated　in　batch　experiments.　Three　biokinetic　models　(Haldane,　Aiba,　and　Yano)　were　used　to　describe　the　experimental　data　with　adjusted　R　2　values　of　0.995,　0.981,　and　0.994,　respectively.　The　K-s　and　K-i　values　for　the　Haldane　kinetic　model　were　2.997　+/-　0.041　and　64.736　+/-　0.023　mg/L,　respectively.　The　maximum　specific　degradation　rate　of　ammonium　(q(max))　occurred　at　the　substrate　(FA)　concentration　of　S-ma(x　)=　13.94　mg/L　with　a　value　of　3.946　+/-　0.021　g/(g　DCW　d)(-1).　The　specific　degradation　rate　of　ammonia　decreased　by　about　50%　(IC50)　at　an　FA　concentration　of　76.31　mg　NH3-N/L,　calculated　using　a　logistic　dose-response　model.　The　real-time　polymerase　chain　reaction　results　revealed　that　FA　inhibited　nitrogen　removal　and　growth　of　the　bacterial　strain　during　heterotrophic　nitrification　by　inhibiting　expression　of　the　amoA　gene.