Broiler carcass quality can be improved by conventional selection techniques. In this regard, an experimental "quality" line (QL) was selected for high breast meat yield. We analyzed the effects of this selection on the dietary lysine requirement in chicks from 0 to 3 wk. Control (CL) and QL chicks were provided ad libitum access to isoenergetic diets containing 20% crude protein but differing in their lysine content (0.75, 0.88, 1.01, and 1.13%). Two-way ANOVA showed a significant effect (P < 0.01) of genotype on body weight, growth rate, feed intake, and weight of Pectoralis major and Gastrocnemius muscles. Conversely, the Sartorius muscle weight was not modified (P = 0.21) by genotype. Lysine deficiency markedly reduced body weight, growth rate, and feed intake, and increased feed conversion ratio (P < 0.001). Low dietary levels of lysine also depressed the weight of Gastrocnemius, Sartorius, and P. major (P < 0.001). The body or muscle weight response to diet lysine concentration depended on the line, with QL chicks appearing less sensitive to lysine deficiency. Consequently, their dietary requirements could be lower. Finally, when weight gain and P. major muscle protein deposition were plotted against lysine intake, QL chicks appeared to be more efficient than CL chicks. The underlying mechanisms responsible for this await clarification.