Poly(trimethylene terephthalate) (PTT) has been polymerized at temperatures between 150 and 220 degreesC as extended chain lamellar single crystals, ca 100 Angstrom thick, by the confined thin film melt polymerization (CTFMP) technique. The crystal structure, determined from electron diffraction patterns and modeling using the CERIUS2 simulation program is compared with that of solution gown crystals from higher molecular weight commercial PTT and PTT polymerized in the hulk at temperatures from 180 to 220 degreesC; the crystal density is highest for the CTFMP material and least for the solution precipitated commercial PTT. The triclinic unit cell of PTT contains two chemical repeat units from a single chain, with lattice parameters of a = 4.53, b = 6.15, c = 18.61 Angstrom, alpha = 97 degrees, beta = 92 degrees, gamma = 111 degrees, rho = 1.448 g/cm(3).