A parallel analysis of the polymorphic behavior and mechanical properties of syndiotactic polypropylene samples with different stereoregularity and crystallized in the helical form and in the trans-planar mesomorphic form, stretched at room temperature and at 4 degreesC, is reported. Oriented fibers show good elastic properties at room temperature, regardless of the crystalline form present in the original fiber. The elastic behavior is associated with reversible polymorphic transitions occurring in the crystalline regions during the stretching and relaxation of fibers. The helical form or trans-planar mesomorphic form present in the fibers transform by stretching into the form III, which transforms back into the helical or trans-planar mesomorphic forms by removing the tension. At 4 degreesC fibers in trans-planar mesomorphic form do not transform into the form III by stretching, and no phase transition is observed upon releasing the tension. Correspondingly, the elastic recovery at 4 degreesC of mesomorphic fibers is significantly reduced in a very small deformation range. These results indicate that both the enthalpic effect, due to the polymorphic transition occurring in the crystals, and the entropic effect, due to the conformational transitions of the amorphous tie chains, contribute simultaneously to the elastic recovery of syndiotactic polypropylene fibers.