Crystals have been grown of two similar peptides that form ion-conducting channels in diphytanoyl phosphatidylcholine bilayers. These crystals were grown by slow evaporation of the organic solvent, 2,2,2-trifluoroethanol. Crystals of one of the peptides have been characterized by X-ray diffraction, and X-ray data have been measured to 2.3 A resolution. Earlier it was proposed that the ion-conducting channels formed by these peptides consist of four peptides associated as a parallel alpha-helical tetramer. On the basis of the space group and unit cell dimensions of the crystals, a packing scheme for the peptide is proposed that is consistent with a tetrameric channel.