Toxins produced by Aeromonas hydrophila strain A10 in the muscle of carp at the site of injection were studied. Carps injected with the crude toxins showed swelling and reddening of the body surface as those injected with the live bacteria. The toxic substance was partially purified from a extract of the necrotized muscles by ammonium sulfate precipitation, ion-exchange chromatography and gel filtration. As a result, it was suggested that α-type hemolysin was implicated in the toxicity. The hemolysin was heat-labile, and inactivated by EDTA, trypsin and papain, but stable at pH value between 4.0 and 11.2