We have found that alamethicin, in the absence of an electric field, modifies both the hydrophilic surface and hydrophobic core of lipid bilayers. As shown by freeze-fracture and X-ray diffraction experiments with multiwalled vesicles, alamethicin increases the fluid space between bilayers by as much as 50 nm, and at the same time perturbs the hydrocarbon regions of the bilayers. For suspensions of gel-state lipid treated with alamethicin, uniformly spaced rows of particles cover the fracture faces and corresponding linear arrays of stain-collecting depressions cover the hydrophilic surfaces. In the liquid-crystalline state, alamethicin induces an irregular granular texture on the fracture faces.