The assumption that the proline residues in feather keratin, which comprise 12 per cent of the total, are periodically located along the polypeptide chain is shown to lead to an essentially unique structure for this fibrous protein. The structure is based on a beta-helix; i.e., an extended chain which coils slowly to form a helix of relatively large pitch. Such helices tend to aggregate by hydrogen bonding to form cylindrical units, which in turn can aggregate further into cable-like structures. This model has been tested with respect to its predictions concerning the x-ray diffraction pattern, infrared spectrum, mechanical properties, and chemical behavior of feather keratin. Preliminary results indicate that it is better capable of accounting for the data than previously proposed structures.