The range of exercise intensities that elicit high fat oxidation rates (FOR) in youth and the influence of pubertal status on peak FOR are unknown. In a longitudinal design, we compared FOR over a range of exercise intensities in a small cohort of developing prepubertal male subjects. Five boys all at Tanner stage 1 (ages 11–12 yr) and nine men (ages 20–26 yr) underwent an incremental cycle ergometry test to volitional exhaustion. FOR curves were determined from indirect calorimetry during the final 30 s of each increment. The same protocol was duplicated annually in the boys as they progressed through puberty. The peak FOR was considerably higher ( P < 0.05) in boys at Tanner 1 (8.6 ± 1.5 mg·kg lean body mass−1·min−1) (mean ± SD) compared with men (4.2 ± 1.1 mg·kg lean body mass−1·min−1). FOR dropped as boys developed through puberty (Tanner 2/3 peak rate = 7.6 ± 0.6 mg·kg lean body mass−1·min−1; Tanner 4 peak rate = 5.4 ± 1.8 mg·kg lean body mass−1·min−1, main effect of Tanner stage; P < 0.05) to the levels found in men (not significant). The exercise intensity that elicited peak FOR was higher in the boys at Tanner 1 [56 ± 6% peak aerobic power (V̇o2 peak)] than in men (31 ± 4% V̇o2 peak) ( P < 0.001). This value tended to decrease by Tanner stage 4 (45 ± 10% V̇o2 peak, main effect of Tanner stage; P = 0.06). We conclude that, compared with men, prepubertal boys have higher relative FOR throughout a wide range of exercise intensities and that FOR drops as boys develop through puberty.