The susceptibility of Atlantic (Salmo salar), sockeye (Oncorhynchus nerka), and chinook salmon (O. tshawytscha) post-smolts to infection with infectious hematopoietic necrosis virus (IHNV) in seawater was determined. Fish were challenged in seawater by cohabitation with fish intraperitoneally injected with IHNV, and by IHNV bath exposure. Atlantic salmon were susceptible to IHNV infection by both cohabitation and bath exposure. Sockeye salmon were infected by cohabitation with viral injected Atlantic salmon, but not by bath exposure. Chinook salmon were refractory to infection. Infected Atlantic salmon exhibited severe petechial and ecchymotic hemorrhages in the viscera, pale livers, bloody ascites, and anemia. This study indicates that IHNV poses a threat to Atlantic salmon reared in seawater and therefore, the introduction of infected fish to netpens should be avoided.