A new form of syndiotactic poly(1-butene), named form I', has been found in fibers of syndiotactic butene-propylene copolymers with propylene concentrations in the range 10-30 mol %. The structure of the new form I' is similar to that of the stable antichiral form I of syndiotactic polypropylene and is characterized by a B-centered packing of chains in 2-fold s(2/1)2 helical conformation in an orthorhombic unit cell with values of axes that depend on the propylene concentration. Propylene units are, indeed, included in the crystals of form I' and values of axes of a = 17.98 angstrom, b = 6.03 angstrom and c = 8.18 angstrom have been determined for the copolymer sample containing 11 mol % of propylene. Disorder in the alternation of right-and left-handed helical chains along the axes of the unit cell is present so that the structure can be described by the statistical space group Bmcm.