The structure of a ring-containing fluoropolymer poly(1,2-difluoromethylene-1,2,3,3-tetrafluoro-4,4-dihydrofuran) has been investigated by X-ray methods. Pressed films are more than 60% crystalline, and small-angle X-ray data show that these contain lamellae oriented with their long axes parallel to the surface. The wide-angle X-ray data show that the chains are perpendicular to the film surface and have an axial repeat of 9.62 Angstrom, probably containing two monomers (an axial advance of 4.81 Angstrom per monomer). Molecular mechanics modeling indicates that the most likely structure contains syndiotactic chains in which the monomers have the three configuration : such chains are ribbonlike, with an axial advance of similar to 4.8 Angstrom, in good agreement with the observed repeat. In contrast, the preferred conformation for the isotactic chain of three monomers is a nonintegral helix with close to five monomers per turn and an axial advance of similar to 4.1 Angstrom/monomer, which is not compatible with the X-ray data. Chains of monomers in the higher energy erythro configuration have monomer repeats in the region of 3.9-4.2 Angstrom and appear unlikely. Thus, the results suggest that the high degree of crystallinity is due to a highly syndiotactic, probably blocky microstructure for the polymer.