A　denitrifying　strain　ZY04　with　a　high　nitrite-accumulating　rate　was　isolated　and　purified　from　activated　sludge　in　a　laboratory-scale　A(2)/O　reactor.　The　strain　was　characterized　and　identified　as　Acinetobacter　johnsonii　by　16S　rDNA　phylogenetic　analysis.　The　sequences　of　the　key　functional　genes　(napA,　nirB,　nirD)　involved　in　partial　denitrification　were　amplified　via　polymerase　chain　reaction,　which　provided　a　basis　for　exploring　gene　expression.　The　effects　of　different　environmental　factors　(C/N　ratio,　pH　and　temperature)　on　the　partial　denitrification　performance　and　transcriptional　levels　of　the　functional　genes　during　the　logarithmic　growth　phase　were　investigated　by　batch　experiments.　The　results　showed　that　the　partial　denitrification　performance　was　optimal　when　the　C/N　ratio　was　5,　the　pH　value　was　6-8　and　the　temperature　was　25　degrees　C.　The　gene　expression　during　the　logarithmic　growth　phase　indicated　the　good　performance　of　partial　denitrification　under　different　environmental　conditions.　All　three　functional　genes　exhibited　the　highest　expression　levels　at　25　degrees　C.　The　results　of　inhibitory　kinetics　analysis　revealed　that　three　biokinetic　models　(Aiba,　Edwards　and　Andrews)　simulated　the　growth　pattern　of　strain　ZY04　inhibited　by　a　single　substrate　(nitrate　or　sodium　acetate)　well.　In　the　double-substrate　inhibitory　model,　five　models　of　nine　combinations　successfully　fitted　the　growth　characteristics　of　the　strain　affected　by　the　double　substrate　of　nitrate　and　sodium　acetate.　The　relevant　semi-saturation　parameters　and　substrate　inhibition　parameters　were　obtained,　and　the　correlation　coefficient　(R-2)　reached　98%.