Composite materials in which the organic host is stiffened by guest particles, are widely used in nature and are produced synthetically by man. Organisms also produce a different type of composite in which the host is a crystal and the guests are macromolecules occluded in an orderly fashion within the crystal. The best studied examples, to date, are biogenic calcite crystals, and in particular those formed by the echinoderms. In vitro experiments with calcite crystals grown in the presence of echinoderm intracrystalline proteins, show that these macromolecules are occluded inside the crystal on specific planes, and their presence alters the mechanical properties of the crystal host. Furthermore, the proteins also influence the crystal textural properties. Model studies using crystals of dicarboxylic acid salts grown in the presence of intracrystalline proteins show that the coherence length is reduced in directions perpendicular to the planes on which the proteins adsorb. We found anisotropic effects in almost all the biogenic calcite crystals we examined. Furthermore, we noted an interesting relationship between the variations in coherence length in the different crystallographic directions and the gross morphology of the single crystal elements, suggesting that these proteins may also function in determining the morphology of the crystal during growth. These novel single crystal-protein composites may be just one example of strategies used in nature for producing materials with special properties.