Aerolysin is a cytolytic toxin which forms channels in the plasma membranes of eucaryotic cells. The protein is secreted by Aeromonas hydrophila as an inactive protoxin. Its stability and water solubility are conferred by its ability to dimerize. Maturation of the protein occurs through proteolytic removal of a C-terminal peptide outside the secreting cell. Although the aerolysin which is so produced is still a dimer, it then has the ability to oligomerize. The oligomer is the active form of the toxin, capable of forming the transmembrane channels that disrupt cells. We review here the present knowledge about the structure of aerolysin in relation to the various steps in channel formation