DEAE-cellulose ion-exchange chromatography separates soluble organic matrix components of three mollusk shells, each from a different taxonomic class, into analogous subfractions. The proteins of all subfractions are enriched in acidic and polar amino acids. In each chromatogram, however, the subfraction which contains the major portion of total protein also contains the highest concentration of aspartic acid. Thus the major components of the soluble organic matrix are aspartic acid-rich proteins. The identification of these proteins in mollusks, together with the known occurrence of aspartic acid and phosphoserine-rich proteins in vertebrate tooth dentin, emphasizes the fundamental importance of such acidic proteins in the processes of mineralization.