Abstract The cytotoxic Aβ fibril is a logical candidate for the entity causing the initiating damage to neurons in Alzheimer’s disease and Down’s syndrome. We have derived a model of binding for the dye molecule, Congo red (CR), to a β-sheet structure of β-amyloid (1–42). This model is based on the crystal coordinates of CR binding to porcine insulin fibrils from Turnell and Finch. Intact insulin is composed of protein dimers and X-ray diffraction studies show that CR intercalates between two insulin monomers at an interface formed by a pair of antiparallel β-strands. The intercalation of CR has disrupted the four main-chain hydrogen bonds between the two β-strands, but they are still tethered with each other through new hydrogen bonds with the CR nitrogen atoms. The CR molecule has been aligned along the homologous stretch of amino acids in Alzheimer β peptide (two molecules in antiparallel distorted or pseudo β-sheet conformation) using the crystal coordinates from the Turnell-Finch paper to arrive at a putative structure for CR binding to Alzheimer’s amyloid fibrils.