Studies using low-resolution fiber diffraction, electron microscopy, and atomic force microscopy on various amyloid fibrils indicate that the misfolded conformers must be modular, compact, and adopt a cross- structure. In an earlier study, we used electron crystallography to delineate molecular models of the N-terminally truncated, disease-causing isoform (PrP Sc ) of the prion protein, designated PrP 27-30, which polymerizes into amyloid fibrils, but we were unable to choose between a trimeric or hexameric arrangement of right-or left-handed -helical models. From a study of 119 all- folds observed in globular proteins, we have now determined that, if PrP Sc follows a known protein fold, it adopts either a -sandwich or parallel -helical architecture. With increasing evidence arguing for a parallel -sheet organization in amyloids, we contend that the sequence of PrP is compatible with a parallel left-handed -helical fold. Left-handed -helices readily form trimers, providing a natural template for a trimeric model of PrP Sc . This trimeric model accommodates the PrP sequence from residues 89 -175 in a -helical conformation with the C terminus (residues 176 -227), retaining the disulfide-linked ␣-helical conformation observed in the normal cellular isoform. In addition, the proposed model matches the structural constraints of the PrP 27-30 crystals, positioning residues 141-176 and the N-linked sugars appropriately. Our parallel left-handed -helical model provides a coherent framework that is consistent with many structural, biochemical, immunological, and propagation features of prions. Moreover, the parallel left-handed -helical model for PrP Sc may provide important clues to the structure of filaments found in some other neurodegenerative diseases.