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The pathogenicity of six Vibrio harveyi strains in tiger prawn, Penaeus monodon, was studied, using both live bacteria and extracellular products (ECP). The organisms originally isolated from diseased penaeids were more virulent using both live bacteria and ECP (LD50, 4.87-8.65 × 104 colony-forming units (cfu) and 1.20-1.51 µg protein g-1 body weight) than the two reference strains originally isolated from either sea water (ATCC 25919; LD50, 3.18 × 106 cfu and 2.70 µg protein g-1 body weight) or diseased Talorchestia sp. (ATCC 14126, 0.418 × 106 cfu and 2.34 µg protein g-1 body weight). Each strain was reisolated from the haemolymph and the hepatopancreas of moribund prawns following each bacterial challenge. Both the live bacteria and the ECPs of the penaeid isolates exhibited stronger proteolytic (caseinase), phospholipase and haemolytic activities than those of the reference strains. These results indicate that there are differences between penaeid and non-penaeid isolates of V. harveyi in pathogenicity and reveal that proteases, phospholipases, haemolysins or exotoxins might play leading roles in the pathogenicity of V. harveyi in the tiger prawn, Penaeus monodon.