Poly(ethylene naphthalate) (PEN) is a polymer with potentially useful industrial properties. During a study of the morphology and properties of drawn PEN fibres it became apparent that an ordered structure existed in some specimens that was not the well-known crystalline form. This structure is considered to be a mesophase in which substantial lengths of individual chains are fully extended but do not pack laterally in crystalline register. The existence of this intermediate phase is strongly dependent upon processing conditions and could have implications for the properties of commercially produced fibres, since it appears to be stable and not easily converted to the crystalline form at elevated temperatures.