We investigated differences in walking (80 m/min) and running (147 m/min) economy [submaximal oxygen consumption (O_2 submax)] between adolescent girls (n = 13; age = 13.3 ± 0.9 yr) and young women (n = 23; age = 21.0 ± 1.5 yr). Subjects were matched for height (158.7 ± 2.9 cm) and weight (52.1 ± 3.0 kg). Anthropometric measures (height, weight, breadths, skinfolds) and preexercise oxygen consumption were obtained on all subjects before submaximal and maximal treadmill exercise. Anthropometric measures were similar between groups, as was maximal oxygen consumption (girls, 47.7 ± 5.2; women, 47.5 ± 5.7 ml · kg¹ · min¹). O_2 submax was significantly greater (P < 0.0002) in girls compared with women during both walking (16.4 ± 1.7 vs. 14.4 ± 1.1 ml · kg¹ · min¹) and running (38.1 ± 3.7 vs. 33.9 ± 2.4 ml · kg¹ · min¹). Preexercise oxygen consumption (4.4 vs. 3.9 ml · kg¹ · min¹) accounted for only a fraction of the differences found in exercise economy. Although heart rate and respiratory frequency were greater in the girls in both walking (118 ± 11 vs. 104 ± 12 beats/min and 31 ± 3 vs. 25 ± 4 breaths/min, respectively; P < 0.002) and running (180 ± 15 vs. 163 ± 17 beats/min and 47 ± 11 vs. 38 ± 8 breaths/min; P < 0.005), this did not likely account for a large part of the difference in O_2 submax between groups.