The elastic constants of poly(glycolic acid) (PGA) crystals are reported on the basis of a commercial software package and the published crystal structure of the polymer. Due to the planar zigzag conformation of the molecular chains, very high elastic anisotropy is found with a tensile chain modulus of 294 GPa and a longitudinal shear modulus for a fibre of 6 GPa. A combination of small and wide angle X-ray scattering and differential scanning calorimetry are used to characterise the structure of highly oriented PGA fibres. The combination of long period data, crystal size and crystallinity measurements suggests a structure similar to the Prevorsek model, with alternating crystalline and amorphous regions along the fibre axis, and layers of amorphous material in parallel. A parallel-series Takayanagi model, using the theoretically calculated chain modulus, is shown to give good agreement with the experimental data in a wide temperature range. (c) 2006 Elsevier Ltd. All rights reserved.