The chemical and stereochemical structure of lenthionine has been established by the X-ray method. Lenthionine crystallizes in the monoclinic system, space group P2₁/a-C⁵_2h, with four molecules of S₅ (CH₂)₂ in the unit cell. Using a three-dimensional Patterson function and minimum functions, all atomic positions were determined. The least squares method was adopted to distinguish CH₂ groups from sulfur by the temperature factors. Thus the three-dimensional molecular geometry has been elucidated to be in a chair form, having C₂ pseudo-symmetry.