Although the result of long-time detail ecological monitoring shows that the pollution of water and sediment is serious, there is no acute toxicity to fish in the aquatic ecosystems of Xuanwu Lake. This was proved by toxicity tests(in situ and in laboratory) and simulation tests (simulated the lake water and sediment). The pathogenic bacteria, Aennnonas hydrophila, were isolated from silver and variegated carp of the lake, and is believed to be the responsible pathogen. From the results of determining germ virulence and observing the symptoms of fish inection it can be concluded that the cause of fish eruptive death is a kind of bacteriosis caused by A. hydrophila. The results of comprehensive analyses have drawn the following conclusion. In the summer when the redox shielding level(Eh, +200mV) is close to the surface of the sediment, the introduction of pathogenic bacteria from the sediment into the water occurs readily. The dominant population, Merismopedia, is not a preference bait for fish. On the contrary, it makes fish struggle along on the verge of starvation during the summer. In addition, terrible pollution of water and sediment may not only destroy the antibody systems of fish but also effect the dynamic of pathogenic bacteria. Therefore, this bacteriosis is an environmental disease.