The crystal structure of the poly(trimethylene sebacate) polyester, PE(3,8), was arrived at through a combined analysis of its electron, powder, and fiber diffraction patterns. PE(3,8) has an orthorhombic unit cell of dimensions a = 5.032, b = 7.532, and c = 31.33 Angstrom (fiber axis) and belongs to the P2(1)2(1)2(1) space group. There are two chemical units per fiber repeat and the two chains within the unit cell are positioned on 2(1) screw axes, parallel to the c direction. The fiber repeat is 2.50 Angstrom shorter than that for an all-trans conformation. The non-trans torsion angles are located in the glycolic moiety of the chemical repeat. A conformational analysis revealed several energy minima which, after minimization, yielded 18 potentially acceptable conformations. The most likely conformation for the -O-(CH2)(3)-O-flexible segment of the polyester is g(+)tg(-)g(+), while the sebacate group is in the all-trans conformation. The final structure and its orientation and position within the unit cell were arrived at by a comparison of the theoretical and experimental intensities recorded on single crystal electron diffractograms and on X-ray diffraction powder patterns. The proposed structure;is confirmed by the small values taken by the conventional R factor, R = 0.108 for 19 powder diffraction arcs and R = 0.136 (R(w) = 0.096) for 12 independent spots on the electron diffraction recording.