A highly crystalline sample of the extra-large-pore, high-silica zeolite UTD-1 has been prepared in fluoride medium. The resulting product (UTD-1F) has a powder diffraction pattern remarkably different from that published for the calcined form of conventionally prepared UTD-1, and it could only be indexed on a monoclinic unit cell. The crystal structure was determined ab initio in the space group P21/c from synchrotron powder diffraction data collected on a textured sample. The resulting structure, with 69 non-H atoms in the asymmetric unit, is by far the largest structure solved directly from powder diffraction data to date. Subsequent Rietveld refinement of the structure in Pc, with 349 positional parameters and 464 geometric restraints, proved to be surprisingly stable and converged with RF = 0.041 and Rwp = 0.134 (Rexp = 0.101). In contrast to the structure reported for calcined UTD-1, no evidence of disorder was found in the UTD-1F framework (structure type code DON). There is a strict up−down altern...