The　relationship　between　algicidal　bacteria　and　harmful-algal-bloom-forming　dinoflagellates　is　understudied　and　their　action　modes　are　largely　uncharacterized.　In　this　study,　an　algicidal　bacterium　(FDHY-03)　was　isolated　from　a　bloom　of　Prorocentrum　donghaiense　and　the　characteristics　of　its　action　against　P.　donghaiense　was　investigated　at　physiological,　molecular,　biochemical　and　cytological　levels.　16S　rDNA　sequence　analysis　placed　this　strain　in　the　genus　of　Alteromonas　in　the　subclass　of　gamma-proteobacteria.　Algicidal　activity　was　detected　in　the　bacterial　filtrate,　suggesting　a　secreted　algicidal　principle　from　this　bacterium.　Strain　FDHY-03　showed　algicidal　activity　on　a　broad　range　of　HAB-forming　species,　but　the　greatest　effect　was　found　on　P.　donghaiense,　which　showed　91.7%　mortality　in　24　h　of　challenge.　Scanning　electron　microscopic　analysis　indicated　that　the　megacytic　growth　zone　of　P.　donghaiense　cells　was　the　major　target　of　the　algicidal　action　of　FDHY-03.　When　treated　with　FDHY-03　culture　filtrate,　P.　donghaiense　cell　wall　polysaccharides　decreased　steadily,　suggesting　that　the　algicidal　activity　occurred　through　the　digestion　of　cell　wall　polysaccharides.　To　verify　this　proposition,　the　expression　profile　of　beta-glucosidase　gene　in　FDHY-03　cultures　with　or　without　P.　donghaiense　cell　addition　was　investigated　using　reverse-transcription　quantitative　PCR.　The　gene　expression　level　increased　in　the　presence　of　P.　donghaiense　cells,　indicative　of　beta-glucosidase　induction　by　P.　donghaiense　and　the　enzyme＇s　role　in　this　dinoflagellate＇s　demise.　This　study　has　isolated　a　new　bacterial　strain　with　a　strong　algicidal　capability,　documented　its　action　mode　and　biochemical　mechanism,　providing　a　potential　source　of　bacterial　agent　to　control　P.　donghaiense　blooms.