Pseudotuberculosis is a bacterial septicaemia caused by Photobacterium damselae subsp. piscicida in several marine fish species.A similar Pseudotuberculosis of Seriola dumerili cultured in cage, broke out in Guangdong province, China. A lot of small typical whitish spots were observed in kidney and spleen of infected fish. The typical whitish spots were diffusely distributed in the whole kidney and spleen, both on the surface and section. These small whitish spots are about 1 millimeter in diameter. In order to diagnose the Seriola dumerili of suspected Pseudotuberculosis, the histopathologic changes of the infected fish and the morphology of the pasteurella picicida were observed under optic and electronic microscopy. A Gram-negative and rod-shaped bacterium was isolated from the spleen and kidney. The rod-shaped bacterium were dyed into dark blue in both end by Methylene blue.The size of the rod-shaped bacteria were approximately 0.8-2.6μm-long and 0.6-1.2μm-wide.The optical microscopy showed that not only scattered bacteria were observed in the blood,but also basophilic bacterial cluster-like structures were observed in the spleen and kidney. These bacterial cluster-like structures are about 1-2 millimetre in diameter. By squashing tissue of the kidney and spleen, there are large numbers of rod-shaped bacterium，which clustered into mission, or diffused distribution. Transmission electron microscopy showed that a small amount of bacteria were in macrophages and a large number of rod-shaped bacteria clustered into mission in the kidney and spleen.The bacterial clusters were surrounded by connective tissue. No virosome were observed in the infected organs by electron microscopy.According to the results above, the disease was diagnosed as Pseudotuberculosis. The main histopathologic changes of the other organs under optic microscope showed as follow: Denaturalization and necrosis were emerged in renal tubule epithelial cell, and lymphocytosis were in interrenal tissue. Number of macrophages in the renal interstitium.A large number of blood cells congested in renal interstitial, and there were basophilic rod-shaped bacteria and bacterial cluster-like structures in renal interstitial. Congestion,lymphocytosis and bacterial cluster-like structures were showed in spleen; Hepatocyte appeared fatty degeneration; The gill lamellar epithelial cells were swollen.Damage to heart included myocardial cell vacuolar degeneration, swelling, rupturing and necrosis; Damage to Intestinal epithelial cells were denaturalization, necrosis and inflammatory cell infiltration. Capillary of cerebral cortex and medulla were congestion. a small amount of nerve cell were degeneration and necrosis.