Scanning X-ray microdiffraction has been carried out across single polymeric fibers using a beam profile of 0.1 mu m across the fiber axis. As-spun and heat-treated poly(p-phenylenebenzobisoxazole) (PBO) fibers have been studied to investigate the influence of heat treatment on fiber structure. Meridional "layer line" scattering intensity is observed to vary across the fiber width. This can be related to variations in the degree of structural order. Through the association of disordered regions with a layered morphology, a fiber model has been developed. The model proposes a three-layer fiber morphology with gradual transitions between layers. The core region can be offset from the geometric center of the fiber, and also be asymmetric. For fitting of the model to experimental data, a Monte Carlo simulation approach is employed. For both PBO fiber types, the fiber model provides a good fit to the experimental results obtained. The modeling results indicate that the heat treatment process causes a number of morphological changes within the fiber. These include a reduction in the thickness of the fiber skin and a distortion of the fiber core. Thus, the morphological core of the heat-treated fiber does not coincide with the geometric fiber center and is highly asymmetric. Several as yet unexplained results are also reported such as a larger core size in the heat-treated fiber type.