The optically active and crystalline poly(alpha-methyl-alpha-ethyl-beta-propiolactone) (PMEPL) shows conformational changes and polymorphic behavior depending on the preparation method of the sample. The melt-crystallized isotactic PMEPL exhibits a monoclinic lattice and an extended planar zigzag conformation. Using computational modeling, we have solved and refined its crystal structure. Both the two-step and single-step procedures were used to evaluate packing energies. The results indicate that two chains with the same chirality and opposite directions pass through a monoclinic unit cell having lattice dimensions a = 9.10, b = 7.44, c(fiber period) = 4.84 Angstrom and beta = 83.1 degrees. The space group is P2(1) with unique axis b. The final crystal structure was confirmed by comparison with electron diffraction data and X-ray powder diffraction spectra. Both procedures result in crystal structures in good agreement with the experiments. The final discrepancy R factors are 0.16 for the flexible model and 0.20 for the rigid model when compared to X-ray data and 0.20 for the flexible model. and 0.23 for the rigid model when compared to electron diffraction data.