Optically active poly(alpha-methyl-alpha-n-propyl-beta-propiolactone) (PMPPL) crystallizes in a 2(1) helical conformation, but its conformational structure and packing symmetry have not been solved and refined yet. On the basis of the rotational isomeric state approximation and the conformational algorithm for polymer helices, optimum conformational models derived from single helices were used as a starting point in building crystal structures. Both intra-and intermolecular interactions were simultaneously optimized. Semiempirical molecular mechanics calculations of an isotactic single chain having fixed experimental helical parameters revealed that the preferred conformation for PMPPL entails the ttg(-)g(-) backbone chain conformation with a g-t side-chain orientation. Crystal models compatible with the observed orthorhombic lattice dimensions (a = 10.6, b = 11.1 Angstrom) were built and refined against electron diffraction data, X-ray powder diffraction spectra, and structure factor calculations. The favored structure contains two 2-fold screw helices packed antiparallel in an orthorhombic lattice with space group P2(1)2(1)2(1)-D-2(4). With the ttg(-)g(-) main chain conformation, both the g-t and tt conformations are possible for the side chain according to the refinement of X-ray structure factors. However, the g-t side-chain conformation shows a better fit than that of the tt conformation with the electron diffraction patterns. The flexible procedure of energy minimization yields distinct values of fiber period for the g-t and tt conformations, e.g., 6.31 and 6.14 Angstrom, respectively. The final discrepancy R factors are 0.20 for the g-t and 0.17 for the tt side-chain models when compared to X-ray data and 0.17 for the g-t and 0.27 for the tt side-chain models when compared to electron diffraction data.