The myelin basic protein (MBP) is the most abundant intracellular protein of the myelin, which forms the electrically insulating sheath of axons of many actively functioning neurons. This protein binds the opposite membranes of the flattened processes of oligodendrocytes and plays a crucial role in myelin compaction. Here we show that MBP is present in amyloid form in the oligodendrocytes in the brain of vertebrates. It forms SDS-resistant insoluble aggregates and clearly colocalizes with Congo Red and Thioflavin S in vivo, ex vivo, and in vitro. The fibrils of MBP extracted from the brain are detected by electron microscopy and exhibit apple-green birefringence after Congo Red staining. We showed that the central region of MBP, spanning amino acid residues 60-119, is responsible for the formation of amyloid fibrils. Based on these data, we present a model in which MBP not only connects the opposite membranes of oligodendrocyte processes but also provides longitudinal amyloid stitching of myelin sheaths. Amyloid fibrils appear to be an ideal natural material for myelin compaction and axon insulation.