The crystallization and solid-state structure of the biobased polyester composed of isomannide and succinic acid (M4) were systematically investigated by changing the crystallization conditions such as casting solvent and crystallization temperature (T-c). It was found from wide-angle X-ray diffraction that M4 had the crystal polymorphism: casting from the chloroform solution gave the alpha-form, whereas thermal annealing at T-c >= 160 degrees C gave the beta-form. A mixture of alpha- and beta-forms was obtained at T-c < 160 degrees C, and the beta-form fraction increased with increasing T-c. The unit cell of the a-form was determined by the fiber diffraction method: a = 0.523 nm, b = 0.913 nm, c (fiber axis) = 2.082 nm, and alpha = beta = gamma = 90 degrees (monoclinic P2(1)), where two 2/1 helices were packed in the unit cell in a parallel manner. M4 chains in the beta-form took the same chain conformation but a different chain packing. The crystallization half-time of M4 drew an inverted bell-shaped curve against T, in which the minimum was ca. 200 s at T-c = 140-150 degrees C.