This paper reports the research results of pathology of dermaulcer caused by Aeromonas hydrophila subsp hydrophila during the overwintering season of Tilapia nilotica. The histopathalngical changes were characterized by the alternative inflammativn. In, ulcerated focus, the skin was inflammatory oedema, necrosis and finally fell away; nucleus of skeletal muscles swelled, moved to the renter, and finally necrosised; muscles fibres swelled anal necrvsised. Liver cells cloudy swelled and fat degenerated. The epithelial cells in the proximal， convoluted tubules of the kindney cloudy swelled, glassy degenerated boundary Lines of ceps became unclear, but the nucleus still remained. The healing of sbeletal muscles in focus was mended through the formation of granulative tissue and regeneration. The healing of epidermis was mended by regeneration. The biochemi-pathological changes were characterized by the signifieant changes of the activities and contezzts of LDH, AIDH and α-GPDH of white xuuscle of diseased fish. Number of bands of electropherotograms decreased, activities decreased or disappeared completely. The contents of muscle prvtein also devreasod significantly.