High levels of mortalities (20%) associated with abnormal swimming have been observed in farmed juveniles of seabass, Dicentrarchus labrax, in Mediterranean coast, southern France. Histopathology was obvious in the brain with nervOUS tissue necrosis and the presence of ceUs containing basophilic granules corresponding to smaU coccoid organisms. Ultrastructural examination revealed these organisms were rickettsia-like organisms morphologicaUy close to Piscirickettsia salmonis