A plasmacytoid leukemia was observed in chinook salmon Oncorhynchus tshawytscha reared in seawater netpens in British Columbia, Canada. The disease was first observed in market-size salmon (2 to 4 kg) and caused high mortality at several facilities. The disease, referred to as marine anemia by fish farmers, is characterized by pallor of the gills due to anemia, enlargement of the spleen and kidney, and ascites. Some affected fish exhibited prominent bilateral exophthalmia. Histological examination revealed massive proliferation of plasmacytoid cells (plasmablasts) in the kidney interstitium, spleen, intestinal lamina propria, pancreas, liver, and heart. Fish with exophthalmos exhibited massive proliferation of the plasmablasts in the periorbital connective tissue, ocular muscles, and choroid gland. In tissue sections, the plasmacytoid cells had large, often lobate of clefted nuclei, and a moderate amount of amphophilic cytoplasm. In Giemsa imprints, cells had a smooth contour, contained a large nucleus, an intense staining cytoplasm, and in some cells a juxtanuclear hof was visible. Electron microscopy and immunohistochemistry revealed the plasmacytoid features of the proliferating cells. The cell contained a well organized, rough endoplasmic reticulum, the cisternae being distended with a lightly granular material. Immunoglobulin was detected in the cells in tissue sections with a goat antitrout immunoglobulin peroxidase stain. Although an infectious etiology (e.g. oncogenic virus) for the disease was suspected, no viruses have yet been detected by cell culture of electron microscopy.