A toxin lethal to fish was purified from extracellular products (ECP) of Edwardsiella tarda by ion exchange chromatography and gel filtration. The purified toxin is a protein having proteolytic activities and lethal toxicity to Japanese eel Anguilla japonica with the LD50 of 1.6μg per g of fish body weight. Molecular weight of the protein was estimated to be 37 kDa by SDS-PAGE. SDS-PAGE, Western blot and immunodiffusion analyses revealed that the 37 kDa protein was present in the ECP and ICC (intracellular components) of virulent strains of E. tarda but not in those of avirulent strains, indicating that this toxin plays an important role in the pathogenicity of E. tarda.