β-Amyloid (Aβ) peptides are the main protein component of the pathognomonic plaques found in the brains of patients with Alzheimer's disease. These heterogeneous peptides adopt a highly organized fibril structure both in vivo and in vitro. Here we use solid-state NMR on stable, homogeneous fibrils of Aβ(10-35). Specific interpeptide distance constraints are determined with dipolar recoupling NMR on fibrils prepared from a series of singly labeled peptides containing 13C-carbonyl-enriched amino acids, and skipping no more that three residues in the sequence. From these studies, we demonstrate that the peptide adopts the structure of an extended parallel β-sheet in-register at pH 7.4. Analysis of DRAWS data indicates interstrand distances of 5.3 ± 0.3 Å (mean ± standard deviation) throughout the entire length of the peptide, which is compatible only with a parallel β-strand in-register. Intrastrand NMR constraints, obtained from peptides containing labels at two adjacent amino acids, confirm the secondary structural findings obtained using DRAWS. Using peptides with 13C incorporated at the carbonyl position of adjacent amino acids, structural transitions from α-helix to β-sheet were observed at residues 19 and 20, but using similar techniques, no evidence for a turn could be found in the putative turn region comprising residues 25-29. Implications of this extended parallel organization for Aβ(10-35) for overall fibril formation, stability, and morphology based upon specific amino acid contacts are discussed.