This study compares the effectiveness of three selected exercises (curl-up, conventional hook sit-up and modified hook sit-up) in building abdominal strength and endurance in upper elementary school girls. Ninety-nine subjects were randomly divided into four groups. The study was designed to determine (1) whether an eight week exercise program can increase abdominal strength and endurance; (2) the most effective exercise; (3) if muscle action intensity affects the results. A cable tensiometer measured abdominal strength and a timed sit-up measured abdominal endurance. An analysis of covariance determined significance. Significant gains in abdominal strength and endurance were shown by the conventional hook sit-up. Further study on the relationship between muscle development and maturation in young children is recommended.