The two crystal structures of poly(butylene-2,6-naphthalate) (PBN) have been determined by X-ray diffraction. The space group is for both forms. The unit cell parameters of the B form are a = 4.55 Angstrom, b = 6.43 Angstrom, c (fibre axis) = 15.31 Angstrom, alpha = 110.1, beta = 121.1 and gamma = 100.6. The major difference in crystal structure between the A and B forms lies in the conformation of four methylene group sequences : <(SG)over bar TGS> for the A form and for the B form.