Aerolysin is one of a large group of bacterial proteins that can kill target cells by forming discrete channels in their plasma membranes. The toxin has many properties in common with the porins of the Gram-negative bacterial outer membrane, including an extensive amount of β-structure, a high proportion of hydrophilic amino acid side-chains and no hydrophobic stretches in the primary structure. It also oligomerizes to produce an insertion-competent state. Aerolysin is secreted as a dimer by members of the Aeromonas family. It binds to a high-affinity receptor on the target cell that has recently been shown to be a glycosylphosphatidylinositol-anchored glycoprotein. Binding is followed by heptamerization to form a structure that we propose contains a β-barrel which can insert into the membrane and produce a channel.