Anorganic unerupted developing teeth and air-dried erupted teeth of the platypus (Ornithorhynchus anatinus) were examined in a scanning electron microscope and in a tandem scanning reflected light microscope. Typically mammalian developing fronts of enamel and dentine were identified in the anorganic unerupted specimens. The developing teeth were particularly small and fragile and the enamel elusive and difficult to examine in the normal way for morphological detail. Prepared fractured surfaces of unerupted specimens revealed preferentially oriented crystallite groups in the enamel generally perpendicular to the developing front and a highly globular, mineralized pattern in the dentine with fine diameter, sparsely distributed dentinal tubules. Although optically homogeneous, the enamel of both developing and mature teeth displayed well-defined incremental lines, radial clefts, crystallite domains of variable size and outline, and fine tubules when examined by high contrast, back-scattered electron imaging. The enamel is prismatic only in part; well-formed, regular prisms not being a primary feature of platypus enamel. This can be related to the variability inherent in the developing surface and the thinness of the enamel layer. No surface was found which could be confidently identified as cementum; those developing surfaces not covered by enamel displaying small calcospherites which elsewhere marked the outer aspect of the dentine.