A plasmacytoid leukemia (PL) has caused mortalities in chinook salmon (Oncorhynchus tshawytscha) reared in seawater netpens in western British Columbia, Canada, since 1988. Kidney or eye tissues from 11 of 13 fish from netpens with clinical PL had reverse transcriptase (RT) activity. This RT activity was associated with virus particles of retrovirus morphology and buoyant density. In a transmission experiment, PL-positive donor fish tissues also had RT activity and virus particles of retrovirus morphology and buoyant density, as did recipient fish tissues following development of the disease 6 weeks postinjection with a tissue homogenate from the donor fish. Kidney and spleen tissues from fish that developed PL following injection with an inoculum that was passed through a 0.22-micron filter, in a separate experiment (M. L. Kent and S. C. Dawe. Further evidence for a viral etiology in the plasmacytoid leukemia of chinook salmon Oncorhynchus tshawytscha. Dis. Aquat. Org., in press, 1992), also exhibited RT activity. The virus particles observed by electron-microscopic examination of tissues or sucrose fractions from PL-positive fish were enveloped and were about 110-nm diameter with a central electron-dense core. Polypeptides of about M(r) 120,000, 80,000, 42,000, 27,000, 25,000, 22,000, and 19,000 were observed when purified virus particles were examined by polyacrylamide gel electrophoresis analysis. Many infectious neoplasms of animals, including fishes, are caused by retroviruses. The evidence in this study shows the presence of a retrovirus in chinook salmon with PL and further suggests a retroviral etiology of the disease. We are tentatively calling this virus salmon leukemia virus.