The structure and organisation of the organic matrix of the cusps of the major lateral teeth of chitons have been examined using conventional and light microscopy in two species, Acanthopleura hirtosa and Plaxiphora albida. The results show that the organisation of the matrix differs between different regions of individual cusps. In addition, there are considerable differences in matrix structure of the same region between the two species. This is particularly so in the case of the transition zone that separates the posterior and anterior compartments of the teeth and which, in A. hirtosa, would appear to be the major factor limiting ion flow between them. The structure of the matrix at the surface is also different to that in the rest of the tooth, presumably in order to toughen the cutting surfac