Abstract: The paper describes the isolation, toxicity as well as physiological and biochemical properties of the pathogenic bacterium on the caudal peduncle of grass carp, The bacterium was identified as a species of the genus Aeromonas. It does not utilize L-arabinose, L-arginine and salicin as sole source of carbon, and does not grow on KCN medium, hydrolyse esculin, or ferment salicin, and it produces gas from glucose. The per cent mole of guanine plus cytosine (G+C Mol%), determined by the method of Marmur, in DNA of representative strain (CR79-1-1) was 60.26 Mol%, which agreed with that of Popoff et al. We, therefore, are of the same opinion that Aeromonas sobria is the causative agent of pedunele disease of grass carp. The tests of medical treatment and inhibition of this disease were also carried out. Results indicate that using a dosage of 0.25 ppm Furazolidoum and 0.125 unit/ml Aureomycin or 2 unit/ml Chloromycetin is more effective. A comparison of the artificial infection and serology of the mentioned bacterium with other puthogenic bacteria that cause diseases on fish body surface was made.