From 2001 to 2002,a new and emergent infectious disease of Ophiocephalus argus occurred in a fishery in Hubei Province,China,with an incidence of 60%～70% and a mortality as high as 100%.The diseased fish showed an enlarged abdomen,the millet-like nodules in internal organs,and the swollen kidney which was composed of 5～10 sarcoma-like bodies in cream or gray-white colour or ulcerated into beandregs-like substance.Light microscopic observation revealed the basophilic or acidphilic inclusions in cytoplasm of the cells and the granulomas,a diffusive chronic inflammation in internal organs.Further analysis under an electron microscope indicated that the intracytoplasmic inclusions were rickettsia-like organisms (RLOs) that are either spherical or coccoid,with variable size,ranging from 0.5～1.5 μm in diameter,and enclosed within membrane-bound cytoplasmic vacuoles.RLO had a central nucleoid region with some fine filamentous structures and an electron-dense granule.Its cytoplasm contained abundant ribosomal bodies.Occasionally,RLO appeared to be divided by binary fission.RLOs were also observed in the homogenized tissue of infected fish.The results suggested that the death of cultured O.Argus was caused by RLO infection.