Polyethylene terephthalate fibres in the drawn or extended state show considerable optical and mechanical anisotropy. In the undrawn state the fibres are essentially isotropic and the degree of subsequent anisotropy in the drawn state may be controlled by varying the amount of extension imposed.A series of polyethylene terephthalate fibres were made with a range of draw ratios, all from one sample of isotropic fibre, and their mechanical and optical properties were measured; the mechanical measurements were mainly those of extensional and torsional moduli whereas the optical properties were those of refractive indices and hence birefringence. An attempt to explain the development of both mechanical and optical anisotropy on a theoretical basis in terms of the orientation of the polymer molecules has met with limited success.