We model oligomers of the Alzheimer's amyloid beta-peptide A beta(1-42) in an implicit membrane to obtain insight into the mechanism of amyloid toxicity. It has been suggested that A beta oligomers are the toxic species, causing membrane disruption in neuronal cells due to pore formation. We use basin-hopping global optimization to identify the most stable structures for the A beta(1-42) peptide monomer and small oligomers up to the octamer inserted into a lipid bilayer. To improve the efficacy of the basin-hopping approach, we introduce a basin-hopping parallel tempering scheme and an oligomer generation procedure. The most stable membrane-spanning structure for the monomer is identified as a beta-sheet, which exhibits the typical strand-turn-strand motif observed in NMR experiments. We find ordered beta-sheets for the dimer to the hexamer, whereas for the octamer, we observe that the ordered structures separate into distinct tetrameric units that are rotated or shifted with respect to each other. This effect leads to an increase in favorable peptide-peptide interactions, thereby stabilizing the membrane-inserted octamer. On the basis of these results, we suggest that A beta pores may consist of tetrameric and hexameric beta-sheet subunits. These A beta pore models are consistent with the results of biophysical and biochemical experiments.