The dentition of the American opossum,Didelphis marsupialis, has been examined using prepared skulls, occlusal casts and, during normal function in chewing, by cinefluorography. The information obtained from these studies has been used in a simulation of the power stroke of mastication on a stereotaxic machine. The directions and amplitudes of the movements used in chewing have been demonstrated.Two types of power stroke are described; a crushing‐puncturing stroke associated with tooth‐food‐tooth contact occurring in the early stages of mastication, and a shearing stroke with tooth‐tooth contact occurring later. The first type produces a rapid flattening of the tips of the cusps, the second a series of striated wear facets on the slopes of the cusps. In the shearing stroke the food is triturated by an anteromedial movement of the paracristid of the lower molars across the metacrista of the upper. Active chewing occurs on one side of the mouth only at any one time and for a considerable number of cycles. The symphysis does not cross the midline during any sequence of cycles. The anteromedially directed power stroke is mediated in part by the mobile symphysis which allows asymmetrical movement of the two dentaries and by a Bennett shift of the mandibular condyles.The similarity between the molars ofDidelphisand those of the early therians, notablyAlphadon, PappotheriumandHoloclemensiais recognized. It is suggested that the information obtained from this study may assist in the interpretation of early therian