The aggregation of proteins into amyloid fibrils is associated with several neurodegenerative diseases. In Parkinson's disease it is believed that the aggregation of a-synuclein (alpha-syn) from monomers by intermediates into amyloid fibrils is the toxic disease-causative mechanism. Here, we studied the structure of a-syn in its amyloid state by using various biophysical approaches. Quenched hydrogen/deuterium exchange NMR spectroscopy identified five beta-strands within the fibril core comprising residues 35-96 and solid-state NMR data from amyloid fibrils comprising the fibril core residues 30-110 confirmed the presence of beta-sheet secondary structure. The data suggest that beta 1-strand interacts with beta 2, beta 2 with beta 3, beta 3 with beta 4, and beta 4 with beta 5. High-resolution cryoelectron microscopy revealed the protofilament boundaries of approximate to 2 x 3.5 nm. Based on the combination of these data and published structural studies, a fold of alpha-syn in the fibrils is proposed and discussed.