The crystal structure of poly(pentamethylene 2,6-naphthalate) (PPN) was determined by using X-ray diffraction and molecular modeling. The unit cell of PPN was found to be triclinic (P (1) over bar space group) with dimensions of a = 0.457 nm, b = 0.635 nm, c = 2.916 mn, alpha = 121.6degrees, beta = 90.4degrees, gamma = 87.6degrees, and the calculated crystal density was 1.311 g cm(-3). The unit cell contains one polymer chain with two repeating units. In the unit cell, the PPN backbone takes gauche/gauche conformation in the middle part of each pentamethylene unit, and two naphthalene rings are in face-to-face arrangement.