Soluble oligomers are potent toxins in many neurodegenerative diseases, but little is known about the structure of soluble oligomers and their structure-toxicity relationship. Here we prepared on-pathway oligomers of the 140-residue protein alpha-synuclein, a key player in Parkinson's disease, at concentrations an order of magnitude higher than previously possible. The oligomers form ion channels with well-defined conductance states in a variety of membranes, and their beta-structure differs from that of amyloid fibrils of alpha-synuclein.