An extracellular lethal toxin produced by Vibrio alginolyticus strain Swy originally isolated from diseased kuruma prawn (Penaeus japonicus) was partially purified by Fast Protein Liquid Chromatography with hydrophobic interaction (Phenyl Sepharose High Performance) chromatography and gel filtration columns. The toxin is an alkaline serine protease, inhibited by phenyl-methylsulfonyl fluoride (PMSF), and showed maximal activity at pH 10, having a molecular weight of about 33 kDa estimated by SDS-PAGE and gel filtration chromatography. In addition, the toxin was also completely inhibited by FeCl2 but partially inhibited by CaCl2, CuCl2, CoCl2, MnCl2, and ZnCl2, and not inhibited by ethylenediamine tetraacetic acid (EDTA), ethylene glycol-bis(beta-amino-ethyl ether) N,N,N',N'-tetraacetic acid (EGTA), iodoacetamide, pepstatin A, sodium dodecyl sulfate (SDS), and N-tosyl-l-phenyl-alanine chloromethyl ketone (TPCK). Both the crude extracellular products (ECP) and the partially purified toxin are lethal for kuruma prawn at LD(50) values of 0.30 and 0.27 mu g protein/g body weight, respectively. The addition of PMSF completely inhibited the lethal toxicity of both the ECP and the partially purified toxin, indicating that this serine protease is a lethal factor produced by the bacterium. The 33-kDa protease is,therefore, suggested to be a new toxic protease produced by V. alginolyticus strain Swy.