The purpose of this study was to investigate the effects of muscle fiber composition and muscle cross-sectional area on muscle strength in isokinetic contractions. Muscle biopsies were taken from the right vastus lateralis of 9 healthy untrained men. Using staining for myofibrillar adenosinetriphosphatase after preincubation at pH 4.3 and 4.6, muscle fibers were classified as ST, FTa, FTb, and FTc. But, FTc fibers were not calculated. Cross-sectional area of the leg extensor muscles were measured by ultrasonic measurements. Maximum strength for the leg extensor muscles were determined as peak torques with constant angular velocity using an isokinetic dynamometer (Cybex II, Lumex, New York). The peak torque values at various angular velocities (30°, 60°, 120°, and 180°/s) indicated significant positive correlation to %FTa fibers, % Area FTa fibers and Total area of FTa fibers. Other fiber types such as ST or FTb, however, were not related significantly to the peak torque values at all angular velocities. Significant positive correlation was observed between peak torque values per unit cross-sectional area of muscle (absolute strength) and % FTa fibers. These results suggest that in untrained men, %FTa fibers play more important role in producing high peak torques thah %FTb and % ST fibers, and that even if muscle cross-sectional areas are equal, muscles with high % FTa fibers can exert high isokinetic strength.