It has been proposed that movements of cells at the base of the developing wing cause the wing to rotate (Yander and Searls, '80a,b). It was suggested that rotation of the wing caused the dorsal muscle mass of the wing bud to become the posterior musculature of the proximal part of the wing. Rotation of the wing was investigated by grafting dorsal myogenic tissue from a wing labeled with tritiated thymidine into the dorsal myogenic region of an unlabeled wing in its original orientation. Grafts were made both with and without their original ectoderm. The location and shape of the graft was recorded at the time of the operation. After a period of growth and morphogenesis, the host wing and adjacent body wall were fixed, sectioned, and prepared as autoradiographs. The location of the grafted cells was determined by reconstructing the host wing and adjacent body wall. It was found that the graft increased in length on the axis of the humerus and did not increase rapidly on any other axis. During stage 25, a deep groove began to form ventral to somite 19 because of ventral and cranial movement of the lateral body wall proximal to the base of the wing. The distal border of this groove was recognized as the posterior margin of the proximal wing. If the graft extended to the base of the wing, the graft participated in the formation of this groove. Grafted cells on the distal border of the groove were on the posterior surface of the wing at the level of the humerus.