"Rotten skin" is one of the bacterial fish diseases, its infection rate reaches to80%, which causes great loss of Nile--tilapia in the wintering ponds in some districtsof China. The major symptom of the diseased fish is that, the shape of erosion on the skinis either circular or elliptic. Most of the erosions appear on both sides of the dorsalfin, sometimes scatter on all parts of the body. In its early stages, the surface of thebody appears to be filled with blood, the scales around them begin to loosen and erectup, gradually it rots to a hole, at last causes the death of the fish. The bacterium wasisolated from the liver and focus of diseased fish, and artificial infection with pure cul-tures shows positive. The bacterium is a short rod 0.5--0.6×0.7--1.4 μm long, withrounded ends, with a polar motile flagellum, nonsporulating, and Gram-negative inits stain reactions. Products gas from glucose, glycerol, galactose and mannitol;Veges--Proskauer reaction positive; oxidizcs gluconic acid. According to morphology,cultural character as well as physiology and biochemisty reactions fit well to character-isties of Aeromonas hydrophila subsp. hydrophila biotype I, which is presented inBergey's Manual of Determinative Bacteriology 8th ed. Aeromonas hydrophila subsp.hydrophila biotype I should be considered as a agent for the "Rotten--skin" disease ofNile-tilapia.