AEROLYSIN is chiefly responsible for the pathogenicity of Aeromonas hydrophila, a bacterium associated with diarrhoeal diseases and deep wound infections¹. Like many other microbial toxins, the protein changes in a multistep process from a completely water-soluble form to produce a transmembrane channel that destroys sensitive cells by breaking their permeability barriers². Here we describe the structure of proaerolysin determined by X-ray crystallography at 2.8 Å resolution. The protoxin (M_r 52,000) adopts a novel protein fold. Images of an aerolysin oligomer derived from electron microscopy have assisted in constructing a model of the membrane channel and have led to the proposal of a scheme to account for insertion of the protein into lipid bilayers to form ion channels.