The organic matrix of the radula teeth of the chiton Acanthopleura hirtosa has been examined by a variety of techniques to identify and partially characterize the major structural component and to elucidate the complex architectural arrangement of matrix fibres. The teeth are initially composed solely of organic material, a major component of which has been identified as the partially deacetylated <latex>$\alpha $</latex>-polymorph of the polysaccharide, chitin. The matrix consists of organic fibres, with significant changes in the organization and distribution of fibres occurring in the early stages of tooth development, presumably in preparation for the onset of mineralization. Organic fibres in the posterior region of the tooth cusp become progressively more sparse and poorly ordered. In contrast, the anterior section of the cusp becomes more structured, consisting of a series of highly ordered rope-like fibres. At high magnification these fibres can be seen to consist of hollow tubules linked by fine strands or bridges. Demineralized mature teeth show the same variation in the fibre density and arrangement. It is suggested both that mineralization occurs on a preformed organic framework of which the fibrous bridges are an integral component and that the organization of the matrix fibres together with the architecturally discrete distribution of minerals contributes significantly to the overall mechanical properties of the tooth.