Abstract.Viral haemorrhagic septicaemia (VHS) is reported for the first time in sea‐water cultured rainbow trout. Heavy mortalities with typical signs and lesions A VHS virus (serotype 1) was isolated from the diseased fish. The mortalities were caused only by the VHS virus and 80 days post transfer of trout to sea‐water the mortalities reached 85%, of the initial population.The disease was experimentally transmitted to rainbow trout, both in sea‐water 3·104pfu/ml of virus or by intramuscular injection of various doses of VHS1(7·1017·104or 7·104pfu per fish). Death occurred in all infected groups and started earlier in sea‐water. Typical signs of VHS were observed in moribund fish. Viral multiplication was demonstrated to have occurred in