The first outbreak of red sea bream iridoviral disease caused by red sea bream iridovirus (RSIV) was recorded in cultured red sea bream Pagrus major in Shikoku Island, Japan in 1990. Since 1991, the disease has caused mass mortalities of cultured marine fishes not only red sea bream but also many other species. The affected fish were lethargic and exhibited severe anemia, petechiae of the gills, and enlargement of the spleen. The causative agent was a large, icosahedral, cytoplasmic DNA virus classified as a member of the family Iridoviridae and was designated as red sea bream iridovirus (RSIV). The genome of RSIV is liner dsDNA and considered to be circularly permitted and terminally redundant like other iridoviruses. The length of physical map of RSIV genome is 112,415bp. An indirect immunofluorescence test with a monoclonal antibody and PCR are commonly used for the rapid diagnosis of RSIV infected fish in the field. For the control of this disease, a formalin-killed vaccine against red sea bream iridoviral disease was developed and now commercially available.