Small-angle synchrotron experiments have been performed in two samples of poly[oxybis (trimethylene) p,p’-bibenzoate], PDTMB. Owing to the very slow transformation of the mesophase of this polymer into the crystal, only the liquid crystalline phase is present in a PDTMB sample quenched from the melt, while about 30% crystallinity is found in the annealed specimen. The results indicate that the layer spacing peak for the mesophase is very close to a crystal diffraction. This seems to be a general fact in thermotropic polybibenzoates. Moreover, no long spacing has been detected in the quenched sample. On the contrary, the annealed specimen exhibits a long spacing centered at about 16 nm, corresponding to a rather small crystal thickness.