In Alzheimer's disease, the amyloid-beta peptide (A beta) is implicated in neuronal toxicity via interactions with the cell membrane. Monomeric A beta (A beta(m)) is intrinsically disordered, but it can adopt a range of aggregated conformations with varying toxicities from short fibrillar oligomers (FO), to globular nonfibrillar oligomers (NFO), and full-length amyloid fibrils. NFO is considered to be the most toxic, followed by fibrils, and finally A beta(m). To elucidate molecular-level membrane interactions that contribute to their different toxicities, we used liquid surface X-ray scattering and Langmuir trough insertion assays to compare A beta(m), FO, and NFO surface activities and interactions with anionic DMPG lipid monolayers at the air/water interface. All A beta species were highly surface active and rapidly adopted beta-sheet rich structures upon adsorption to the air/water interface. Likewise, all A beta species had affinity for the anionic membrane. A beta(m) rapidly converted to beta-sheet rich assemblies upon binding the membrane, and these aggregated structures of A beta(m) and FO disrupted hexagonally packed lipid domains and resulted in membrane thinning and instability. In contrast, NFO perturbed membrane structure by extracting lipids from the air/water interface and causing macroscale membrane deformations. Altogether, our results support two models for membrane-mediated A beta toxicity: fibril-induced reorganization of lipid packing and NFO-induced membrane destabilization and lipid extraction. This work provides a structural understanding of A beta neurotoxicity via membrane interactions and aids the effort in understanding early events in Alzheimer's disease and other neurodegenerative diseases.