Four experiments were performed with carp (Cyprinus carpio) in ponds of 200 m2 to test the effects of lysine supplementation of feeds with a low protein content. The initial mean weight of the carp and their daily rations were equal in all the ponds of the same experiment. In experiments 1 (400 fish/pond, 3 replicates) and 2 (200 fish/pond, 4 replicates), three diets were tested: A. 30% crude protein (1.7% total lysine); B. 25% crude protein + 0.5% lysine-HC1(1.7% lysine); C. 25% crude protein (1.3% lysine). In both experiments diet B resulted in weight gains equal to diet A, 20% higher protein retention and 20% less nitrogen discharge per kilogramme gain. Diet C lagged 10% in gain and excreted 10% more nitrogen than B. In experiments 3 and 4 (200 fish/pond, 4 replicates) the feed protein was reduced further and 5 diets were tested: A, B, C, similar to the former experiments, D. 20% crude protein + 1.0% lysine-HC1 (1.8% total lysine), E. 20% crude protein + 0.5% lysine-HC1 (1.4% lysine). In both experiments the results of groups A, B and C were parallel to experiments 1 and 2. Diets D and E resulted in lower growth compared to diet C (significantly in experiment 4) and increased body fat content. In conclusion, 30% protein in carp feeds, of which half the protein was soybean protein, could be decreased to 25%, by replacing soybean meal by grains, and supplementing lysine and methionine to the level of the 30% control. Growth was not impaired and nitrogen discharge into the environment was reduced approximately by 20%. Further feed protein reduction to 20%, with lysine supplementation, impaired growth rate below that of the unsupplemented 25% control diet, although total N-excretion was reduced even more.