The aim of this study was to evaluate the short term effects of an exercise program on abdominal and back muscles resistance and hamstring flexibility among schoolchildren. Fifty eight healthy schoolchildren were divided into two groups: experimental (15 males and 14 females; mean age 12.2±1.8 years) and control (14 males and 15 females; mean age 11.9±2.1 years). The intervention was performed twice a week, on nonconsecutive days, during six weeks. The duration of each session was fifty minutes and they were composed by warming up aerobic exercises, isotonic and isometric abdominal and back muscle exercises and hamstring stretching. The intervention effects were evaluated through Kraus-Weber test (abdominal resistance), modified Biering-Sørensen test (back muscle resistance) and sit and reach test (hamstring flexibility). Data were analyzed by mixed-design two-way ANOVA, with one between-subjects and one within-subject (time) factors. The alpha level was set at P0.05. The experimental group improved back muscles resistance and hamstring flexibility, but not abdominal muscles resistance. There was a significant interaction between groups and time for back muscles resistance (P=0.018) and for hamstring flexibility (P=0.017). The 6-week training period showed positive results in improving back muscles resistance and hamstring flexibility of schoolchildren.