Maximal aerobic power was measured for 5-6 successive years on 47 Japanese school girls starting from the ages of 9 and 13 years. The subjects were divided into two groups. Nine girls (Group-K) were trained from the ages of 9 to 14 years under a training program. Aerobic power for Group-K was measured from the ages of 9 to 16 years with one year interval. Thirty-eight girls (Group-F) were the ordinary school girls of junior and senior high school. Aerobic power for Group-F was measured from the ages of 13 to 18 years with one year interval. Small increase in aerobic power for Group-K was observed between the ages of 9 and 12 yr. Their aerobic power increased significantly from 1.53 1/min to 2.34 1/min between the ages of 12 and 14 yr. Aerobic power for Group-F was 1.67 1/min and 2.03. 1/min at the ages of 13 and 18 yr, respectively. The peak value of 2.04 1/min was obtained at the age of 17 yr. The individual growth curves of aerobic power for Group-F were classified into 3 types based on the characteristics of the curves. Fifteen of 38 subjects showed almost unchanged pattern (Type-A) during the observation period. Aerobic power in Type-A averaged 1.75 1/min and 1.87 1/min at the ages of 13 and 18 yr, respectively. Eleven subjects showed fluctuating pattern (Type-B) during the observation period. Twelve subjects showed gradually increasing pattern (Type-C) from the age of 13 to 18 yr. The average value for Type-C increased from 1.51 1/min at 13 yr old to 2.31 1/min at 18 yr old. Ten subjects in Type-C voluntarily attended extracurricular sports club activities for girls throughout six school years. On the other hand, fourteen subjects in Type-A did not belong to any extracurricular sports club. The following matters might be concluded in the present study : 1) The training effect on aerobic power might be unclear before the age of 12 yr. 2) The rate of increase in aerobic power was significantly greater in trained group (Group-K) compared to that in ordinary school girls (Group-F) between the ages of 12-13 to 14-15 yr. 3) Physical activities in extracurricular sports club seemed to facilitate a gradual increase in aerobic power during the period of junior and senior high schools. 4) Aerobic power reached to its peak at the age of 15-16 yr for the girls who did not attend any physical activities in sports clubs.