A replicated experiment was initiated to investigate the response of 2 populations of chickens, divergently selected for high and low O2 consumption, to varying protein levels. A 3 × 4 factorial arrangement of four isocaloric protein diets (16, 18, 22, 26%) and three levels of intake (100%, 80%, and 60% of ad libitum) were randomly assigned to 24 pens, resulting in 2 pens per treatment combination for each line. Two hundred and forty chicks from each line were then randomly assigned to the 24 pens, 10 chicks per pen. Criteria measured on a per pen basis were weekly body weight gains, daily feed intake, metabolizable energy (ME) values of the diets, and changes in carcass moisture, nitrogen, fat, and energy. Statistical analysis indicated that feed efficiency and weight gains of the low O2 birds were greater at all protein levels studied than the high O2. Carcass composition data indicated that the low O2 birds gained more lean and less fat tissue than the high O2 birds both totally and as a percentage of dry matter gain. Low O2 birds were also more efficient in the utilization of nitrogen, but there was no difference between the lines in the efficiency of ME utilization. The low O2 birds were slightly superior in net energy utilization, as would be expected since they were depositing more lean tissue. Selection for low oxygen consumption, therefore, has resulted in a faster growing, leaner, and more efficient bird.