Biomineralized structures are hybrid composites formed and stabilized by the close interaction of the organic and the inorganic phases. Crayfish are good models for studying biomineralization because they develop, in a molting-mineralization cycle, semi-spherical mineralized structures referred to as gastroliths. The organic matrix of these structures consists of proteins, polysaccharides, and lipids. Chitin is the main polysaccharide and is concentrically arranged as fibrous chitin-protein lamellar structures. Although several proteins and low-molecular weight phosphorylated components have been reported to be involved in gastrolith mineralization, the occurrence and role of proteoglycans have not been fully documented. We have immunologically analyzed the proteoglycans in gastrolith matrix extracts and histological cross-sections of the gastrolith, and the forming epithelium during premolt and postmolt stages. The results indicate that gastroliths contain proteoglycans that have dermatan-, chondroitin-4- and 6-, and keratan sulfate glycosaminoglycans. These macromolecules are closely associated with the mineral phase of the gastrolith and are easily removed by decalcification procedures. There is also evidence to indicate that epithelial secretion of some of these molecules is temporally regulated during the molting cycle. However, the precise role of these macromolecules in the calcification and stabilization of the amorphous calcium carbonate phase of the gastrolith remains to be established.