Small-angle X-ray scattering was carried out on as-spun and heat-treated poly( p-phenylene benzobisoxazole) (PBO) fibre. While as-spun PBO fibre exhibits only an equatorial streak, generally attributed to elongated voids or fibrils, the heat-treated fibre shows in addition a radial four-point pattern. The long period and radial spacing of this pattern are 28 and 15 nm, respectively. The spacings do not vary with heat-treatment temperature. As the angle defined by the plane normal to the fibre axis and the X-ray beam is increased from 0 degrees to 30 degrees to 60 degrees, the four-point pattern gradually changes to a two-point pattern. The measured long period in the tilted fibre is a projection of that in the untilted fibre. Various possible morphological models are discussed.