The biogenesis of inorganic/organic composite materials such as bone typically involves the process of templated mineralization. Biomimetic synthesis of bone-like materials therefore requires the development of organic scaffolds that mediate mineralization of hydroxyapatite (HAP), the major inorganic component of bone. Using phage display, we identified a 12-residue peptide that bound to single-crystal HAP and templated the nucleation and growth of crystalline HAP mineral in a sequence- and composition-dependent manner. The sequence responsible for the mineralizing activity resembled the tripeptide repeat (Gly-Pro-Hyp) of type I collagen, a major component of bone extracellular matrix. Using a panel of synthetic peptides, we defined the structural features required for mineralizing activity. The results support a model for the cooperative noncovalent interaction of the peptide with HAP and suggest that native collagen may have a mineral-templating function in vivo. We expect this short HAP-binding peptide to be useful in the synthesis of three-dimensional bone-like materials.