Combined wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) studies using synchrotron radiation were carried out to investigate the structural development during in-situ solution spinning of poly(p-phenylenebenzobisoxazole) (PBO) in poly(phosphoric acid) (PPA). WAXD patterns indicated that the structure in the extruded PBO/PPA dope before coagulation (in water) possessed lyotropic "biaxial nematic" nematic order involving PBO-PPA complex as the mesogenic unit. A different biaxial nematic order in pure PBO was found to develop immediately upon coagulation in water. When the coagulation time exceeded 30 min, the complex PBO/PPA structure completely disappeared, and well-oriented PBO crystals were formed. The meridian peaks exhibited long streaks, indicating a translational disorder of PBO molecules in the fiber direction. SAXS patterns of the PBO fiber after coagulation showed equatorial streaks with no further maxima observed, which is attributed to a microvoid structure.