A more complete description of the morphology of poly(p-phenylene terephthalamide) (PPTA) in both as-polymerized polymer and various fibre forms is developed by the utilization of a number of methods (dark-field transmission electron microscopy, X-ray diffraction, solid state n.m.r. spectroscopy, and partial isotopic exchange of accessible amide moieties). This description focuses primarily on the effect of lateral crystallite dimensions on the microscopic dynamic structure and the concomitant macroscopic accessibility properties. The observation of a heterogeneous dynamic structure reflects the imperfect structural consolidation during the processing of PPTA in both as-polymerized polymer and fibre forms. The imperfect microcrystalline structure formed during processing is shown to be the origin of macroscopic accessibility for small molecules and also provides a plausible structural basis for biological accessibility.