Study Design: Prospective single-group repeated-measures design. Objectives: To quantify electromyographic (EMG) muscle activity of the infraspinatus, teres minor, supraspinatus, posterior deltoid, and middle deltoid during exercises commonly used to strengthen the shoulder external rotators. Background: Exercises to strengthen the external rotators are commonly prescribed in rehabilitation, but the amount of EMG activity of the infraspinatus, teres minor, supraspinatus, and deltoid during these exercises has not been thoroughly studied to determine which exercises would be most effective to achieve strength gains. Methods and Measures: EMG measured using intramuscular electrodes were analyzed in 10 healthy subjects during 7 shoulder exercises: prone horizontal abduction at 100° of abduction and full external rotation (ER), prone ER at 90° of abduction, standing ER at 90° of abduction, standing ER in the scapular plane (45° abduction, 30° horizontal adduction), standing ER at 0° of abduction, standing ER at 0° of abduction with a towel roll, and sidelying ER at 0° of abduction. The peak percentage of maximal voluntary isometric contraction (MVIC) for each muscle was compared among exercises using a 1-way repeated-measures analysis of variance (P<.05). Results: EMG activity varied significantly among the 7 exercises. Sidelying ER produced the greatest amount of EMG activity for the infraspinatus (62% MVIC) and teres minor (67% MVIC). The greatest amount of activity of the supraspinatus (82% MVIC), middle deltoid (87% MVIC), and posterior deltoid (88% MVIC) was observed during prone horizontal abduction at 100° with full ER. Conclusions: Results from this study provide initial information to develop rehabilitation programs. It also provides information helpful for the design and conduct of future studies.