alpha-Hemolysin, secreted from Staphylococcus aureus as a water-soluble monomer of 33.2 kDa, assembles on cell membranes to form transmembrane, heptameric channels. The structure of the detergent-solubilized heptamer has been determined by X-ray crystallography to 1.9 Angstrom resolution. The heptamer has a mushroom-like shape and measures up to 100 Angstrom in diameter and 100 Angstrom in height. Spanning the length of the molecule and coincident with the molecular sevenfold axis is a water-filled channel that ranges in diameter from similar to 16 to similar to 46 Angstrom. A 14 strand antiparallel beta-barrel, in which two strands are contributed by each subunit, defines the trans-membrane domain. On the exterior of the beta-barrel there is a hydrophobic belt approximately 30 Angstrom in width that provides a surface complementary to the nonpolar portion of the lipid bilayer. The extensive protomer-protomer interfaces are composed of both salt-links and hydrogen bonds, as well as hydrophobic interactions, and these contacts provide a molecular rationalization for the stability of the heptamer in SDS solutions up to 65 degrees C. With the structure of the heptamer in hand, we can better understand the mechanisms by which the assembled protein interacts with the membrane and can postulate mechanisms of assembly.