By transmission electron microscopy, the ultrastructural pathological changes of major internal organs, including liver, spleen and kidney. in rickettsia-like organisms (RLOs) infected fish, Ophiocephalus argus C. have been described, and the pathological mechanism has been discussed in this paper. Observation showed that RLOs host cells were swollen, electronic density of cytosplasm was low, organelles were swollen and ruptured, and numerous RLOs entered into intercellular spaces together were swollen and ruptured cells. RLO debris and degenerated RLOs were detected in some RLOs host cells. The tissue cells were swollen, organelles were scattered and seldom. Besides vacuolar degeneration and creare-broken, some necrosis changes such as non-shaped electronic dense inclusions were observed in swollen mitochondria. The rough endoplasmic reticula enlarged, ruptured. and numerous ribosomes sloughed from those membrane. Lysosomes in a part of cells increased I the concentric-arranged myelin bodies were found in cytoplasm. The nucleus swelled severely, with intra-nuclear myelin-like figures and intra-nuclear inclusions. pkynosis or karyolysis was also seen in some cells.The ultrastructural pathological changes of liver were observed mainly in hepatocytes, phagocytic cells, endothelial cells of blood vessels and connective tissue cells near to blood vessels. The nucleus of hepatocytes showed intra-nuclear myelin-like figures and intra-nuclear fatty inclusion, The marked and larger fatty-drops were also seen in hepatocytic cytosplasm. The ultra-Structural pathological changes of spleen were found mainly in haematopoietic cells, reticular cells, phagocytic cells and endothelial cells of blood vessels, which was similar to those of the liver. RLOs were usually detected in the cytosplasm of macrophage a special macrophage, which engulfed 3 similar to 4 degenerated cells, was also observed in the spleen. The most severe ultrastructural changes were noted in the kidney. The marked organelles rupture and extensive karyolysis occurred usually in RLOs infected cells of kidney.Based on the ultrastructural pathological observation, the three mechanisms on the pathogenesis for the snakeheaded fish 0. argus) infected by RLOs may be suggested: (i)Rupturing cell ultrastructure and interfering with normal metabolisms of host cells by large quantities of RLOs multiplication; (ii)Damaging membrane system directly by releasing toxin of RLOs; (iii)Resulting in thrombi and disseminated intravascular coagulation (DIC) by causing the ultrastructural pathological changes of the endothelial cells of blood vessels.