In October 2006, a similar nocardiosis of snakehead(Ophicephalus argus), cultured in pond, broke out in Guangdong Province, China. Infected fish showed distension of the abdomen, with compact granulomas diffusely distributed in the visceral organs. More acute lesions such as ulcers and muscle necrosis, necropsy revealed multiple yellowish white, firm nodules, 1-2 mm in diameter, exist throughout the kidney, liver, head kidney, spleen, gill and branchial arch. In order to diagnose the snakehead of suspected nocardiosis, the histopathologic changes of the fish and the morphology of the Nocardia sp were observed under optic and electronic microscopes. Histopathologically, typical chronic granulomatous lesions appeared in those organs, including the kidney, liver, head kidney, spleen, gill, branchial arch, heart, intestines and ovary. Granulomas nodules have a threelayer structure. The center of granulomas nodules is caseous necrosis, necrotic area is surrounded by macrophages an d epithelioid cells，and fibroblast and fibrocyte lie in extreme layer. A lot of filamentous bacteria, branching, occasionally beaded, were visible in necrotic centers. These bacteria were acid fast when stained with ZiehlNeelsen. Under electronic microscope, there were a lot of rodshaped bacteria, no virosome in the infected organs. The sizes of the rodshaped bacteria were approximately 3.5-6.5 μm long and 0.45-6.8μm wide. According to the results above, it was diagnosed as nocardiosis. The main histopathologic changes of the other organs under optic microscope showed as follows: Denaturalization and necrosis emerged in renal tubule epithelial cell, and lymphocytosis were in interrenal tissue; congestion and lymphocytosis was showed in spleen; Hepatocyte appeared fatty degeneration; The gill lamellar epithelialcells were swollen, and cardiac muscle fibers were swollen and ruptured; Damage to skeletal muscle included skeletal muscle cell vacuolar degeneration, swelling,rupturing and necrosis; Damage to epidermic cell included denaturalization, necrosis and inflammatory cell infiltration.