The high silicon content of certain radular sites of the Patellacea (Mollusca, Gastropoda) is shown by infrared absorption spectrums to be fixed in the form of the mineral opal. Opal and goethite mineralize the cusps and bases of the teeth. The presence of opalized base plates only in the families Acmaeidae and Lepetidae appears to be of taxonomic significance. Minimum values of the volume of fixation and of the turnover rate of opal by the Patellacea are calculated to assess the role of this previously neglected taxon in biological fixation of silica in the oceans. The significance of these organisms points to the much needed study of the silica transport system in tissue-grade Metazoa.