We have studied an epizootic outbreak occurred in August, 1987, in adult rainbow trout reared in freshwater at Shou county, shanxi Province, China. A total of 12 strains were isolated from the livers, kidneies and spleens of the affected and moribund fish and fish feed and were asssigned to Aeromonas punctata. These strains resulted to be pathogenic for rainbow trout. Their extracellular products (ECP) with thermolability casein protease, thermostable haemolysin also have thermolability lethl effect on fish. The analysis of physical and bacteriological parameters of the water revealed that the epizootic appeared to be "stress" mediated since the mortality occured when the temperature of water reach 20 ℃. Having found the pathogens in putrid forage indicated the virulent agent sourse. These were confirmed by the dropped mortality when the feed were changed or the water temperature dropped. According to our experiment, injecting 4.79mg streptomycin/kg of fish and 1.92mg doxycyclini Hydrochloridi/kg of fish can inhibited the disease. Light microscope studies of the liver, spleen and kidney sections revealted that the intraperitoneal injections of the ECP can produced histopathological signs which associated with the motile Aeromonas infections. It was observed that the swellen hepatic cells, destruction of the eudothelial and reticular cells of the ellipsoids and renal tubules. This observation substantiates the cytolytic and fibrolytic capacities of A. punctata for destroying host defence systems and may contribute to its invasive powers.