The rigid polymer poly(p-phenylene benzobisthiazole) (PBZT) was crystallized from dilute solution. Electron microscopy showed that, on quenching, flat fibrils several nanometers thick were produced. Subsequent heat treatment in a solvent changed the morphology from fibrillar into “segmented ribbon” structure. Isothermal crystallization at a temperature of about 30°C below the dissolution temperature, in general, resulted in aggregation of rod crystals. The polymer chains were oriented normal to the rod crystals. The width of the rod crystal increased with average molecular length, but saturated to a value much smaller than the average molecular length. In the shorter molecular length range, the rod crystals clustered in a fanned-out manner, while with a medium molecular length (ca. 70–120 nm), all rods crystals in a cluster aligned parallel to each other and were of the same length. With longer molecular length (more than ca. 180 nm), the rod growth slowed because of small diffusion constants of molecular chains to the growing face. Based on observation of the morphology and the crystallization process, an isothermal crystallization mechanism is proposed. Because of the rigidity and wide length distribution of polymer chains, the chain ends were inevitably included within the crystals, resulting in crystal defects such as axial shifts, lattice curvatures, and edge dislocations, all of which were observed directly by lattice imaging.