Solution-grown, chain-folded lamellar crystals of poly-beta -propiolactone (PPL) have been crystallized isothermally from cyclohexanone at 55 degreesC. The lathlike crystals have been studied by transmission electron microscopy (imaging and diffraction), atomic force microscopy, and X-ray diffraction. A new Crystal structure, the gamma -structure; has been discovered. Using the experimental diffraction data and computerized modeling the structure has been refined, including the detailed geometry of the adjacent re-entry folds. The chain is in an all-trans or 1-fold helix conformation and the structure consists of a two-chain, C-faced, orthorhombic unit cell with the following parameters: a = 0.700 +/- 0.002 nm, b = 0.490 +/- 0.002 nm, and c (chain axis) = 0.493 +/- 0.002 nm. The setting angles, with respect to the a axis, are +/- 51.5 degrees for the corner and center chains, respectively. The lamellae are 5 nm in thickness and the chains run orthogonal to the lamellar surface, although there is evidence for slight c-axis shearing of the chain-folded sheets. The general fold direction is along the a axis (long axis of the crystal) and the chain folds,successively in the (110) and (1 (1) over bar0) planes. Comparisons are made with previous structures for PPL fibers and related chain-folded polymers.