The purpose of this investigation was to determine whether aerobic fitness (VO2max) differed between black (N = 40) and white (N = 53) adolescent girls who were similar in age (13.5 yr) and percent body fat (24.6%). Expired gases were collected continuously while each girl performed a standard Bruce protocol to volitional exhaustion on a motorized treadmill (TM). Heart rates (HR) were measured during the exercise testing via telemetry. Fat-free mass (FFM) was estimated with total body electrical conductivity (TOBEC). Average (+/- SD) maximum HR (black = 194 +/- 7; white = 198 +/- 8) and respiratory exchange ratios (black = 1.17 +/- 0.08; white = 1.22 +/- 0.09) did not differ between subject groups. Aerobic fitness was significantly lower (P < 0.01) lower in black versus white girls when VO2max was expressed relative to body weight (31.8 +/- 5.8 vs 38.5 +/- 6.8 ml.kg(-1).min(-1)) and body weight raised to the 0.67 power (120.9 +/- 19.5 vs 138.5 +/- 20.7 ml.kg(-0.67).min-1). Treadmill time to exhaustion was significantly less (P < 0.01) in the black (8.49 +/- 1.30 min) versus white (9.41 +/- 1.60) subjects. Also, black subjects demonstrated less ability to utilize O2 during maximal exercise at a given FFM. This suggests the black girls' FFM contained a smaller percentage of skeletal muscle mass that could be utilized during treadmill exercise. It is possible that lower aerobic fitness values seen in the black girls are related to a combination of anatomical, physiological and/or behavioral factors.