The dentine and enamel of a newly discovered well-formed tooth of a Tertiary monotreme, Obdurodon sp., was examined for ultrastructural information relevant to the phylogenetic interpretation of monotremes. Obdurodon sp. presents a unique combination of mammalian structural features in the fractured enamel surfaces examined: tubules in great number; prisms of small diameter; a substantial and coherent inter-prismatic component and, in the middle third enamel, inter-row sheet; a simple non-decussating prism course; a predominantly pattern 1 prism packing in the innermost enamel; and a predominantly pattern 2 prism packing in the middle third enamel; and non-prismatic outer enamel. Although the particular combination of structural characteristics suggests that monotremes represent a possibly unique and relatively plesiomorphic group of mammals, many of the features are very similar to those seen in multituberculates. If multituberculates and monotremes are monophyletic, it would help to explain what has otherwise been a puzzling zoogeographic pattern in the distribution of each group.