Poly(delta -valerolactone), (PVL) crystals in the form of chain-folded lamellae were prepared by isothermal crystallization from a 2-methylbutane-2-ol solution. Wide-angle and small-angle X-ray diffraction data, obtained from PVL lamellae sedimented to form oriented mats, were supplemented with morphological and structural data from electron microscopy, both imaging and diffraction. The diffraction signals index on an orthorhombic unit cell With the parameters a = 0.747 +/- 0.002 nm, b = 0.502 +/- 0.002 nm, and c (chain axis) = 0.742 +/- 0.002 nm. Similar unit cell parameters were obtained from crystals grown from 1-octanol and also from drawn melt-pressed films. The evidence supports a model containing two antiparallel chain segments in the unit cell. The c value of 0.742 nm is appropriate for an all-trans or onefold helical backbone conformation for the straight stems. Possible slight perturbations at the ester units from the all-trans backbone conformation are discussed. Computerized modeling was used to optimize the adjacent-reentry folded structure. The setting angles, with respect to the a axis, are +/- 58 degrees for the corner and center chains. The lamellae are 7.26 +/- 0.05 nm thick, and the chains run orthogonal to the lamellar surface. The chains fold in the diagonal (110) and (1 (1) over bar0) planes in an alternating fashion. The X-ray diffraction data suggest that a proportion of adjacent paired antiparallel entities, or hairpin units, are c-axis-sheared, and a relationship to the results obtained from drawn films is discussed. A brief comparison is also made with related polymer structures.