Flexibacter (=Cytophaga) psychrophila was originally isolated from diseased coho salmon Oncorhynchus kisutch in Washington State, USA, in 1948. Until the mid 1980's, the condition had only been reported as being a wide-spread pathogen of cultured salmonids in the northern states of the USA and in Canada. However, the same bacterium has been recognized since 1984 in western and central Europe as the causative agent of a serious systemic disease in farmed rainbow trout Oncorhynchus mykiss fry capable of causing high mortalities. Typical disease signs in affected fry include anaemia, splenomegaly, a white, fragile intestine and haemorrhagic, protruding anus. Fingerlings and larger fish show less obvious signs such as blindness, skin lesions on the dorsal fin orflank and abnormal swimming behaviour. Hatcheries usually treat affected fish with oxytetracycline, although this is not effective against the chronic form of the disease.