In the course of a routine dissection of the anterior region of the arm an accessory muscle was observed in the flexor aspect of the right arm of a 64-year-old male cadaver. This accessory muscle, which was considered to be the third head of the biceps brachii muscle originated from the inserting tendon of the deltoid muscle. The muscle gave rise to three tendons as it approached the cubital fossa. Two of these tendons inserted to the bicipital aponeurosis whereas the third tendon inserted to the ulnar head of the pronator teres muscle. Dissection of the forearm on both sides revealed that the palmaris longus muscles was absent bilaterally. Mechanical effects of the insertion tendons of the accessory head of the biceps brachii on the median nerve, especially during the supination of the forearm, are discussed. For radiodiagnostic procedures, MRI images of the accessory head were obtained in the coronal and axial planes.