Bacterial kidney disease (BKD) or Dee disease was first described from wild Atlantic salmon, Salmo salar L., caught in two Scottish rivers, the Aberdeenshire Dee and the Spey. In 1935 similar lesions were described in rainbow trout, Salmo gairdneri Richardson, in a North American hatchery, but it was not until 1956 that the causative bacterium was cultured. The causative agent is a small rod shaped bacterium called Renibacterium salmoninarum. It is non-motile, does not produce spores and stains blue with Gram's stain (i.e. it is Gram-positive). It is a very slow growing species commonly taking 10-21 days to produce colonies on agar medium. It can remain viable for some weeks in fresh and sea water without significant amounts of nutrients. Currently 16 countries have reported outbreaks of BKD in farmed stocks of salmonids. The first occurrence of BKD in farmed rainbow trout in Scotland was reported in 1976 and it has now affected 19 rainbow trout farms. At least 43 recurring outbreaks have been recorded. BKD was first reported in farmed Atlantic salmon in 1980. At the present time 18 salmon farms have been infected, 16 in sea water. BKD was added to the list of notifiable fish diseases in Great Britain in 1978.