This study describes shoulder muscle activity during the butterfly stroke. Upon hand entry, the deltoids and rotator cuff muscles demonstrated activity as the humerus was abducted, extended, and externally rotated. The rhomboids and upper trapezius were also active, retracting and upwardly rotating the scapula, which positioned the glenoid for the humerus. During propulsion, the pectoralis major and latissimus dorsi generated power. The subscapularis and teres minor were active to control humeral rotation. The serratus anterior helped to pull the body over the arm by reversing its origin and insertion. The posterior deltoid completed humeral extension at the end of propulsion and began to lift the arm out of the water. Then, the middle and anterior deltoids fired with the supraspinatus and infraspinatus to abduct and externally rotate the arm. The scapular muscles were also active, retracting the proximal portion of the scapula while protracting and upwardly rotating the distal tip. The glenoid then provided a platform for the humerus. Overall, the serratus anterior and the subscapularis maintained a high level of activation throughout the stroke; thus, these muscles were highly susceptible to fatigue and vulnerable to injury.