The crystal structure of poly(tert-butylethylene sulfide) (PTBES) determined in 1977 by Matsubayashi et al. (Macromolecules 1977, 10, 996) is reexamined. Single crystals, of an unusual triangular shape, and epitaxially crystallized samples have been obtained and investigated by electron microscopy and electron diffraction. All the morphological and diffraction evidence indicates that PTBES has a "frustrated" crystal structure, i.e., a packing in which the azimuthal settings differ for the 3-fold helices. The crystal structure thus determined significantly improves that initially proposed for PTBES, within the trigonal crystal lattice determined by Matsubayashi et al. PTBES also illustrates the full range of experimental evidence that indicates the frustrated nature of the structure : characteristic diffraction patterns of single crystals and of epitaxially crystallized films (or of fibers) and triangular morphology of the single crystals.