A rickettsia-like organism was isolated from diseased Atlantic salmon Salmo salar in Norway. Because of morphological and serological similarities to the type strain the suggested name of the organism is Piscirickettsia salmonis. The bacterium is considered the most probable cause of a systemic disease diagnosed in 51 farms along the west coast of Norway. Most of the cases occurred in the autumn of 1988. The disease was only recorded in smolts after exposure to sea water and cumulative mortality has been low. In 63% of fish with gross lesions examined during outbreak of disease in 14 of the affected farms, the typical macroscopic finding was a normal coloured liver with white, circular, sometimes haemorrhagic loci. Of fish with gross lesions, 35% showed pale gills, a yellow, mottled liver, and haemorrhages scattered throughout the skeletal muscles, perivisceral fat, the stomach wall and the swimbladder. Histomorphological changes were most often observed as necrosis and granulomatous inflammation in the liver. Intracellular, intravacuolar bacteria-like inclusions with an affinity for phagocytic host cells were observed. Transmission electron microscopy revealed individual or paired organisms enclosed in membrane-bound vacuoles.