Abstract. – To better understand the neural control and behavioral output of the highly flexible octopus arm, we have characterized the arm’s neuromuscular system using light and electron microscopy to examine the organization of muscle groups and pattern of innervation. Simulations and indirect calculations were used to analyze the morphometric results. Transverse and longitudinal muscle cells have similar dimensions (~1.2 mm x ~8-10 μm) and shape (fusiform). The longitudinal muscle cells are closely packed and embedded in a 3-dimensional collagenous mesh. In the transverse muscles, bundles of muscle fibers are arranged inside loose connective tissue but, contrasting with previous reports, the orientation of these bundles is not necessarily restricted to the transverse plane of the arm. Trabeculae crossing the longitudinal muscles are transversely organized muscle fibers between two thick collagenous layers. As predicted from physiological studies, and differing from other invertebrates, the muscle cells are innervated via localized synaptic junctions. The functional significance of this organization is discussed.