Feeding experiments with semipurified diets containing free amino acids were conducted to determine the quantitative lysine and methionine requirements of long-snout catfish (Leiocassis longirostris Gunther), and practical diets were used to examine the effects of different animal protein levels and supplementation of lysine and methionine on growth and feed utilization. The results were shown below: 1. Free amino acids could be utilized by long-snout catfish. 2. Lysine requirement of fingerlings was found to be 2.59% of the diet or 6.54% of the dietary protein, and methionine requirement to be 0.98% (cystine was 0.04%) of the diet or 2.43% of the dietary protein. 3. Both deficiency and excess of lysine and methionine could reduce growth and feed utilization. 4. Significant increase in GOT activities of liver and decrease in free arginine levels in serum were observed as the dietary lysine levels increased. Content of haemoglobin increased significantly as levels of dietary methionine increased. GPT activities of liver, the content of urea in serum, the ratio of carcass weight/body weight and nutrient composition of fish body were not affected by lysine and methionine markedly. 5. Decrease in dietary animal protein levels significantly reduced growth and feed utilization. 6. Growth and feed utilization were significantly improved by supplementation of lysine and methionine only when the dietary animal protein levels were above 70%. Protein digestibility coefficient increased after the supplementation only at 30% and 50% animal protein levels. The supplementation had no marked effect on energy retention. 7. Supplementation of lysine and methionine in fingerlings, I~+-year-old and II~+-year-old fish were efficient. Based on the results of experiments, the optimum lysine and methionine were estimated to be 6.54% and 2.43% of the dietary protein, and the levels of animal protein should not be lower than 70%. Supplementation of lysine and methionine in practical diets could not only improve growth and feed utilization but also reduce the cost of diets.