The formation of amyloid fibers and their deposition in the body is a characteristic of a number of devastating human diseases. Here, we propose a structural model, based on X-ray diffraction data, for the basic structure of an amyloid fibril formed by using the variants of the B1 domain of IgG binding protein G of Streptococcus. The model for the fibril incorporates four beta sheets in a bundle with a diameter of 45 A. Its cross-section, or layer, consists of four strands, one strand from each sheet. Layers stack on top of each other to form the fibril, which has an overall helical twist with a periodicity of about 154 A. Each strand interacts in a parallel fashion with the strands in the layers above and below it, in an infinite beta sheet. Some geometric features of this model and the logic behind it may be applicable for constructing other related cross-beta amyloid fibrils.