The first fish paramyxovirus was isolated from normal adult Chinook salmon returning to a coastal hatchery in Oregon in the fall of 1982. Subsequently, the virus was isolated from other stocks of adult Chinook salmon and one stock of adult coho salmon in California, Oregon, Washington and Alaska, leading to its designation as the Pacific salmon paramyxovirus (PSPV). The slow-growing virus can be isolated from tissues and ovarian fluids of healthy adult fish returning to spawn and apparently causes no clinical signs of disease or mortality. In 1995, a different and widely disseminated paramyxovirus was isolated from farmed Atlantic salmon in Norway and was designated as Atlantic salmon paramyxovirus (ASPV). Although this virus caused no disease or mortality when injected into juvenile Atlantic salmon, ASPV has been associated with proliferative gill inflammation in sea-reared yearling fish; however, additional infectious agents may be involved in the etiology of the condition. Sequence analysis of PSPV and ASPV isolates using the polymerase gene established their placement in the family Paramyxoviridaeand has shown the two viruses to be closely related but sufficiently different from each other and from other known paramyxoviruses to possibly represent new genera within the family. The viruses can be diagnosed by isolation in cell culture with final confirmation by molecular methods. Other paramyxovirus-like agents have been observed or isolated from rainbow trout in Germany, from seabream in Japan associated with epithelial necrosis, from turbot in Spain associated with erythrocytic inclusion bodies and buccal/opercular hemorrhaging and from koi and common carp associated with gill necrosis in the European Union.