In order to evaluate potential therapeutic targets for treatment of amyloidoses such as Alzheimers disease (AD), it is essential to determine the structures of toxic amyloid oligomers. However, for the amyloid beta-protein peptide (A beta), thought to be the seminal neuropathogenetic agent in AD, its fast aggregation kinetics and the rapid equilibrium dynamics among oligomers of different size pose significant experimental challenges. Here we use ion-mobility mass spectrometry, in combination with electron microscopy, atomic force microscopy, and computational modeling, to test the hypothesis that A beta peptides can form oligomeric structures resembling cylindrins and beta-barrels. These structures are hypothesized to cause neuronal injury and death through perturbation of plasma membrane integrity. We show that hexamers of C-terminal A beta fragments, including A beta(24-34), A beta(25-35) and A beta(26-36), have collision cross sections similar to those of cylindrins. We also show that linking two identical fragments head-to-tail using diglycine increases the proportion of cylindrin-sized oligomers. In addition, we find that larger oligomers of these fragments may adopt beta-barrel structures and that beta-barrels can be formed by folding an out-of-register beta-sheet, a common type of structure found in amyloid proteins.