The anatomy and internal architecture of the jaw musculature in Didelphis marsupialis, the American opossum, was studied using a combination of dissection and thick sectioning techniques. Since the purpose of this investigation was to provide detailed anatomical information as a basis for subsequent functional studies of jaw activity, all muscles associated with normal feeding and ancillary oral behaviour are described. These muscles are the temporal, masseter, pterygoids, digastric, mylohyoid, the remaining suprahyoid muscles and part of the extrinsic tongue musculature. In mammals, the jaw muscles medial to the superficial masseter are classically regarded as the temporal, masseter and zygomatico-mandibular; however, no structural justification for such a division can. be found in Didelphis. With the exception of the outermost layer of the adductor mass which is differentiated as a discrete superficial masseter, the temporal and masseteric part of the adductor musculature is a single unit converging from an extensive origin on bone and fascia to insert onto the coronoid process or its associated tendon. This musculature is described as consisting of three parts: an external adductor originating from the temporal fascia, the zygomatic arch and the masseteric fascia and inserting onto the external surface of the coronoid process, its tendon and the ramus of the lower jaw; an internal adductor originating primarily from the wall of the cranium and inserting onto the inner surface of the coronoid process; and a posterior adductor, the fibers of which pass anteriorly from the cranium posterior to the temporo-mandibular joint to insert onto the posterior border of the coronoid process and the most posterior part of its tendon. The fibers in each part have a different orientation but are not separated into discrete muscles. This division is for descriptive purposes only and no homologies are implied with the similarly named muscles of reptiles. The superficial masseter is a large, fan-shaped muscle extending from a tendinous origin on the maxilla to the inferior surface of the inflected mandibular angle where it has a thick, fleshy insertion. The remainder of the adductor musculature in the opossum consists of a very small external and a thick internal pterygoid muscle. The former inserts into the articular capsule of the temporo-mandibular joint as well as into the condylar neck. The latter has a long, almost linear cranial origin extending posteriorly from the palate toward the temporo-mandibular joint. The fibers pass inferolaterally to insert on the upper surface of the inflected angle. The anatomy of the suprahyoid muscles in the opossum is essentially the same as in eutherian mammals. All the muscles gain part, if not all, of their attachment to the hyoid through a thick, crescentic tendon formed by the fusion of the central tendons of both digastrics. No definite conclusions can be drawn as to the exact function of these muscles on the anatomical evidence alone. However, their position, internal architecture and relative size are suggestive: the external and internal adductors probably have the dual function of suspending the lower jaw from the cranium and adducting the jaw against resistance. The nearly horizontal orientation of much of the posterior adductor is evidence that it can, in addition, act as an effective retractor, with the superficial masseter as its antagonist. In addition to protracting the mandible, the superficial masseter may have a role in producing lateral movement in conjunction with the pterygoids or the adductors. Finally, the suprahyoid musculature in Didelphis probably functions, as in other mammals, to control the movement of the hyoid apparatus, the larynx and epiglottis, and the lower jaw relative to the hyoid. In addition, the mylohyoid, geniohyoid and genioglossus have an important action in elevating and depressing the floor of the mouth and the tongue.