Electromyograms recorded by three or four fine-wire bipolar electrodes inserted along a transverse line across the right tensor fasciae latae of ten normal adults (five men and five women, twenty to thirty-six years old) during a standardized series of exercises and locomotor activities demonstrated different functional roles for the anteromedial and posterolateral fibers of the muscle. Although anatomical dissections in six normal cadavera demonstrated that the force of contraction of the tensor fasciae latae could be transmitted to the knee by the iliotibial tract, the observed activity of this muscle can be explained completely in terms of its action at the hip. The anteromedial fibers have a greater mechanical advantage for hip flexion than do the posterolateral fibers, whereas the posterolateral fibers possess a better mechanical advantage for hip abduction and internal rotation. During walking the anteromedial fibers were electromyographically silent, whereas the posterolateral fibers were active near heel-strike. For all subjects, increased locomotor velocity (during jogging, running, and sprinting) was associated both with increased activity of the anteromedial fibers near toe-off apparently to assist deceleration of extension and acceleration of flexion of the thigh, and with increased activity of the posterolateral fibers at heel-strike.