To identify the key stages in the amyloid fibril formation we studied the aggregation of amyloidogenic fragments of A beta peptide, A beta(1625), A beta(31-40), and A beta(33-42), using the methods of electron microscopy, X-ray analysis, mass spectrometry, and structural modeling. We have found that fragments A beta(31-40) and A beta(33-42) form amyloid fibrils in the shape of bundles and ribbons, while fragment A beta(16-25) forms only nanofilms. We are the first who performed 2D reconstruction of amyloid fibrils by the Markham rotation technique on electron micrographs of negatively stained fragments of AP peptide. Combined analysis of the data allows us to speculate that both the fibrils and the films are formed via association of ring-shaped oligomers with the external diameter of about 6 to 7 nm, the internal diameter of 2 to 3 nm, and the height of similar to 3 nm. We conclude that such oligomers are the main building blocks in fibrils of any morphology. The interaction of ring oligomers with each other in different ways makes it possible to explain their polymorphism. The new mechanism of polymerization of amyloidogenic proteins and peptides, described here, could stimulate new approaches in the development of future therapeutics for the treatment of amyloidrelated diseases.