The sites of replication of infectious hematopoietic necrosis virus (IHNV) in infected tissues were detected in fingerling rainbow trout Oncorhynchus mykiss by in situ histologic techniques following immersion infection. Virus antigens in tissues were detected by a neutralizing mouse monoclonal antibody and a one-step anti-mouse biotin-streptavidin conjugated to horseradish peroxidase. The efficiency of infection and virulence of the virus determined by mortality rates showed high virulence of the selected IHNV isolates, and viral replication in individual fish showed that virus content of the fish increased rapidly from the second day to the seventh day postinfection. The earliest viral lesions following infection were detected in the epidermis of the pectoral fins, opercula, and ventral surface of the body. Virus lesions became evident in kidneys on the third day. By the fifth day, when there was a significant increase in virus titer, foci of viral replication were detected in gill tissue and in the anterior internal tissues below the epidermis. Subsequently, extensive virus replication and tissue destruction were observed in the spleen, dorsal adipose tissues, ventricle, and pseudobranch. Replication in the liver, the muscularis layers of the digestive tract, and the general body musculature followed later. These infection experiments indicated that the epidermis and gills of fish constitute important sites of early IHNV replication.