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The conformation of the polypeptide melittin in lipid membranes as determined by Raman spectroscopy is a bent alpha-helix formed by the mainly hydrophobic residues 1–21, and a nonhelical COOH-terminal segment of the hydrophilic residues 22–26. Fluorescence quenching experiments on residue Trp19 reveal that all COOH-termini are located on that side of a vesicular membrane to which melittin was added. By means of fluorescence energy transfer between unmodified and modified Trp19 residues, melittin is shown to aggregate in membranes predominantly in the form of tetramers. These and previous results on the location and orientation of melittin permit the development of a model for the structure of melittin tetramers in membranes. The hydrophilic sides of four bilayer-spanning helices face each other to form a hydrophilic pore through the membrane.