Oriented fibres and films of a number of synthetic polypeptides have been prepared, and examined by X-ray diffraction. It is shown that the polypeptides can exist in a folded ($\alpha $) and an extended ($\beta $) configuration. The observations on the $\alpha $ form are consistent with the presence of a twofold screw axis in symmetrical polypeptides, and lead to a value of about 5$\cdot $5 angstrom for the true fibre repeat distance. The fibre period in the copolymers is shown to be greater than 5$\cdot $26 angstrom. It is considered that the X-ray evidence favours a ribbon-like molecule, in agreement with the structure shown in I. The $\beta $ forms of the polypeptides give X-ray diagrams similar to those of $\beta $ proteins, and probably consist of extended polypeptide chains. It is shown that the polypeptides can undergo an $\alpha \rightleftarrows \beta $ transformation similar to that encountered in the fibrous $\alpha $ proteins. The medium from which the peptide is regenerated has a very important influence in determining which form is obtained. Formic acid appears to be unique in producing the $\beta $ modification of polypeptides which swell or dissolve in this liquid. A mechanism for this transformation is suggested. A number of copolymers containing glycine have been examined. These appear to exist mainly in the $\beta $ form. The similarities in the behaviour of synthetic polypeptides and fibrous proteins are discussed.