Juvenile chinook salmon ( Oncorhynchus tshawytscha) were fed casein-gelatin-based diets containing graded levels of potassium (0.01–1.20%) for 10 weeks to determine their dietary potassium requirement. The fish were able to sequester potassium directly from the water but they were unable to meet their requirement without additional dietary potassium. Signs of potassium deficiency included anorexia, convulsions, tetany, and death. Although the potassium levels were not replicated in this study, growth-response data suggest that a dietary potassium concentration of 0.8% should produce maximum growth. Whole body potassium saturation (3200 ppm wet fish weight) occurred at a dietary potassium concentration between 0.6 and 1.2% of the diet. Whole body sodium and magnesium were significantly elevated in fish fed potassium-deficient diets.