Endothelin, a potent regulator of vasoconstriction and hypertension, is a naturally produced peptide of 21 amino acids containing two disulfide bonds. We have crystallized endothelin from humans using the vapor-diffusion technique, characterized the crystals by X-ray diffraction analysis, and have collected the X-ray intensities to a resolution of 1.8 A. The crystals, which demonstrate physical properties similar to most protein crystals and have a comparable solvent content, are hexagonal prisms that frequently grow to lengths of 400 microns and widths of 150 microns. The space group of the crystals is P6(1)22 (or P6(5)22), with a = 27.4, c = 79.6 A. There is one molecule of endothelin in the asymmetric unit of the crystals.