AbstractIntracellular inclusions of alpha-synuclein are the neuropathological hallmark of progressive disorders called synucleinopathies. Alpha-synuclein fibrils are associated with transmissive cell-to-cell propagation of pathology. We report the structure of an alpha-synuclein fibril (residues 1-121) determined by cryo-electron microscopy at 3.4Å resolution. Two protofilaments form a polar fibril composed of staggered β-strands. The backbone of residues 38 to 95, including the fibril core and the non-amyloid component region, are well resolved in the EM map. Residues 50-57, containing three mutation sites associated with familial synucleinopathies, form the interface between the two protofilaments and contribute to fibril stability. A hydrophobic cleft may have implications for fibril elongation, and inform the rational design of molecules for diagnosis and treatment of synucleinopathies.