In June 1984, fry of Atlantic salmon, Salmo salar L., (0-3 g mean weight), being reared in a hatehery operated by the University of Washington, began to die and infectious haematopoietic necrosis (IHN) was diagnosed. A total of 40 000 fry were being held in 11 tanks. Mortality due to IHN was noticeable in three tanks, losses being heavy in one and moderate in two. Gross examination of the fish fai