Unfixed fractured dentinal surfaces of a number of different mammalian species were examined both indirectly, by means of a single-stage replica technique, in a transmission electron microscope and directly in a scanning electron microscope. In both instances, so that the third dimension might be appreciated, stereo-pair electron micrographs were prepared by tilting the specimen between exposures. The structural features normally present in dentine - intertubular dentine, peritubular dentine, tubules and their branches - were clearly evident in all the species examined and details of their structure were observed. The appearance of intertubular dentine varied according to the presence or absence of a distinct fibrillar phase while evidence of such a component to the peritubular dentine was restricted to isolated instances. Various and distinct fibrillar conformations were found associated with the tubule walls, the inner aspect of which most often presented as a smooth surface. This surface was, in appearance, almost identical to the peritubular dentine and where the latter was present merged almost imperceptibly with it. Lateral branch tubules of various kinds were clearly demarcated, one very small variety appearing to remain confined to peritubular dentine.