Protein amyloid formation proceeds through a number of different stages. Oligomeric species observed at early stages have aroused particular interest because of evidence for their involvement in cytotoxic processes such as membrane permeabilization. It is unclear whether these oligomers are obligate precursors to fibrils or represent "dead-end" species that impede fibrillation. Because of the many interconverting species present during amyloid formation, it is important to study the process as non-invasively as possible. Small angle X-ray scattering (SAXS) measurements allow us to monitor structural changes in solution for a population of different species over time. Here, SAXS was used to provide a detailed structural description of the fibrillation of the 29 residue peptide hormone glucagon at pH 2.5 from the monomer and early oligomers to mature fibers. Investigation of the pseudo-equilibrium behavior in the lag phase before fibrillation at several concentrations showed that glucagon is present in a monomeric form below about 5.1 mg/mL, while larger oligomers with average aggregation numbers of about three and seven, are formed at 6.4 and 10.7 mg/mL, respectively. Applying several modeling tools to the experimental data, it is shown that the early oligomerization states can be described as associations between glucagon molecules. After the lag phase, a short rod-like protofibril (radius of ~16 A and length >300 A) is formed and subsequently grows to N1000 A in length and assembles into long triple-bundled mature fibers. The protofibril shares many features with the elongated oligomer proposed to be the structural nucleus for insulin fibrils. We propose that on-pathway fibrillar intermediates share this elongated shape that easily allows them to be incorporated into mature fibrils. This contrasts with the annular shape, which is suggested to be involved in cytotoxic membrane permeabilization and may represent a dead-end species off the fibrillar pathway.