The determination of the crystal structure of the bacterial protein proaerolysin provided the first view of a pore-forming toxin constructed mainly from beta-sheet. The structure that was obtained and subsequent crystallographic and biochemical studies have together allowed us to explain how the toxin is transformed from a water-soluble dimer to a heptameric transmembrane pore. Recent discoveries of structural similarities between aerolysin and other toxins suggest that the structure/function studies we have made may prove useful in understanding the actions of a number of pore-forming proteins.