Either simultaneous or separate dietary deficiencies of vitamin E and selenium in Atlantic salmon during first 4 weeks of feeding caused twice the mortality shown in fish fed both supplemental vitamin E (0.5 IU/g dry diet) and selenium (0.1 mug/g). Subsequent dietary repletion with both vitamin E and selenium significantly reduced mortality during the following 2 weeks. Larger salmon (0.9 g initial mean weight), with vitamin E deficiency with or without selenium resulted in the following deficiency signs: extreme anemia, pale gills, anisocytosis, poikilocytosis, elevated plasma protein, exudative diathesis, dermal depigmentation, in vitro ascorbic acid-stimulated peroxidation in hepatic microsomes, yellow-orange liver color, yellow-brown intestinal contents, enlarged gall bladder distended with dark green bile, low vitamin E in carcass and hepatic tissue, muscular dystrophy, increased carcass fat and water, and a response to handling characterized by a transitory fainting with interruption in swimming. A deficiency of dietary selenium suppressed plasma glutathione peroxidase activity. Supplemental selenium with vitamin E significantly increased tocopherol activity in hepatic, but not carcass tissues. Supplements of both vitamin E and selenium were necessary to prevent muscular dystrophy.