Abstract: The histopathological changes of various organs(brain,eye,gill,heart,head,kidney,liver,hepato-pancreas,spleen,kidney,intestine and ovary) in rickettsia-like organisms(RLOs) infected fish,Ophiocephalus argus C. have been reported,and the pathological mechanism has been discussed in this paper. The gross pathological characteristics were that cream-coloured nodules,a chronic granulomatous inflammation under microscopy,were observed on internal organs of infected fish,which had diagnostic significance. Because of extensive tissue necrosis and marked fibroblast proliferation,5—10 giant sarcoma2like bodies with a distinctive boundary were seen on the surface of severe infected kidney. The blood vessels of internal organs showed fibrin thrombi,mixed thrombi and disseminated intravascular coagulation(DIC), and larger areas of tissue and cells showed severe degeneration, necrosis and extensive lysis. Eosinophilic intracytoplasmic inclusion2laden cells,which were mainly epithelial cells and phagocytic cells,situated mostly in the epithelium of gill,the endotheliumof blood vessels and the connective tissue near to the blood vessels of gill and internal organs. The intracytoplasmic inclusions could result in swelling and rupturing of endothelial cells of blood vessels. The histopathological changes of various organs observed in experimentally infected fish with abundant RLOs were similar to those in naturally infected fish. The marked necrosis and larger granulomas were found in brain tissue,fibrin thrombi were also observed in blood vessels of brain membrane. The central necrotic area of granulomas in eye was larger. The gill lamellae were swollen and ruptured,epithelial cells showed necrosis and sloughed. The myocarditis and pericarditis were seen in heart. The severe changes were noted in the kidney. The kidney changes were characterized by the replacement of normal haematopoietic tissue with a large number of granulomas in different size. The renal tubules showed widespread degeneration. The renal corpuscles swelled,the glomerlus congested. The cavities of renal capsule were enlarged in which there were a large number of sloughing epithelial cells. The periphery tissue of kidney showed marked necrosis and extensive lysis. Though the pathological changes in the head kidney were light to those of the kidney,marked granulomas were seen. The granulomas in liver were mainly situated at the periphery of organ and the pathological lesion was mainly fatty degeneration. The epithelial cells in hepato2pancreas were swollen,the acinar cavities of pancreas disappeared. Spleen lesions were similar to those of the kidney. The intestine mucosa thickened,larger granulomas were observed in tunica muscularis with a distinctive boundary and thinner fibrosa. The connective tissue in ovary proliferated,and marked granulomas were also seen in ovary.