1. Methionine and total sulfur amino acid (TSAA) requirements of rainbow trout (Salmo gairdneri) were investigated by feeding graded isosulfurous levels of l- and dl-methionine, l-cystine, and the free acid and calcium forms of methionine hydroxy analog (MHA).
2. Added cystine did not promote growth, survival or prevent cataracts.
3. l-methionine produced fastest growth, followed by dl-methionine, CaMHA and free acid MHA.
4. Trout fed CaMHA gained 85.7 and 92.3% as much as those fed l-methionine and dl-methionine.
5. Within each experiment, the level of L-methionine isomer that prevented cataracts was constant (1.86 g/100g protein in experiment (1), 1.45 in experiment (2) and was lower than for maximum growth (2.89 and 2.15 g) regardless of methionine source.