Methionine and cystine requirements of rainbow trout (Salmo gairdneri) were studied by supplementing a 35% protein semipurified basal diet with these amino acids at graded levels in a factorial design. In experiment 1 (initial average weight of fish, 1.5 g), methionirie levels were 0.30, 0.45, and 0.60% of the diet and cystine levels were 0.04, 0.15, 0.30, 0.45, and 0.60%. In experiment 2 (initial average weight of fish, 8.8 g), methionine levels were 0.55, 0.75, and 0.95% and cystine levels were 0.04, 0.08, 0.12, 0.16, 0.24, and 0.32%. Methionine requirement was estimated to be between 0.55 and 0.75% in the presence of adequate dietary cystine and the cystine requirement was about 0.30% with a diet marginally deficient in methionine. The requirement for both sulfur amino acids was, therefore, between 0.85 and 1.05% of the diet or 2.50 and 3.00% of the protein. Molar efficiency of conversion of dietary methionine to cystine was highest (80%) when the diet was deficient in methionine; this efficiency decreased as levels of dietary methionine increased. Bilateral cataracts occurred in methionine‐deficient but not in cystine‐deficient trout.
