The measurement of the elastic constants of several highly oriented thermoplastic polymer fibres is described. The method makes use of the hot-compaction process, developed and patented in this laboratory, which enables a solid section of highly oriented polymer to be produced from an aggregate of highly oriented fibres. As only a small fraction of the original fibre is melted and recrystallized during the process, the compacted materials offer a unique opportunity for measuring fibre properties in the bulk. An ultrasonic immersion technique is used to measure the elastic properties of the compacted materials, from which the properties of the polymer fibres are inferred. The experimentally determined fibre elastic properties have been compared with other oriented polymer materials to assess any similarities in elastic anisotropy between different methods for producing fibre orientation, and compared with theoretical upper limits for the fibre elastic properties based on theoretical estimates for the polymer crystal unit cell appropriately averaged for hexagonal symmetry using the aggregate model.