Syndiotactic polypropylene (sPP) was melt-quenched in an ice-water bath and directly drawn at 0 degreesC up to a draw ratio of 6. The oriented sample was kept in the cold bath for 10 days under tension, and then analyzed at room temperature either fixed or after releasing tension. The fully extended fiber showed by X-ray diffraction essentially the trans-planar crystalline form III. Upon relaxation, the sample transformed in high proportion to the trans-planar mesomorphic form, free of reflections of helical crystal forms, and became elastic. In this process, there is a reduction of both crystallinity and crystallite orientation. Since the crystalline and mesomorphic chain conformation is trans in both the stretched and relaxed fiber, the elastic behavior in the present sample does not involve a conformational transition in the crystalline domains. At variance, in the infrared spectrum of the relaxed but not of the stretched sample, one of the bands diagnostic of the helical conformation appears. Thus, when the fiber is unhooked, chains tend to disorder reversibly and some segments adopt the helical conformation, even if they must reside in the amorphous component connecting the more ordered mesomorphic domains. Reversible crystallization of the mesophase and of the partially oriented, intercrystalline, amorphous chains into highly oriented metastable trans-planar form III appears to be responsible for the elastic behavior of sPP fibers in the present study. Models of the structural organization in the trans-planar mesophase and the implications of our results with respect to the general mechanism leading to the elastic behavior of sPP are discussed.