Electron diffraction data are used to obtain the structure solution of a large-pore, high-silica zeolite, SSZ-48 that contains an occluded organic structure directing agent. The structure is determined by electron diffraction refinement and is confirmed by high-resolution transmission electron microscopy and Rietveld refinement of powder synchrotron X-ray data. The structure is found to contain a one-dimensional pore system circumscribed by 12 tetrahedral atoms (12 MR), SSZ-48 is the most complex three-dimensional material to be solved at atomic resolution using electron diffraction methods and illustrates the power of electron diffraction data for resolving the structures of materials that form crystals too small for standard single-crystal X-ray analysis.