The crystallization of the trans-planar mesomorphic form of syndiotactic polypropylene by quenching the melt at 0 degreesC is investigated as a function of the stereoregularity of the samples. The formation of the trans-planar mesomorphic form at 0 degreesC is followed as a function of the permanence times at 0 degreesC by X-ray diffraction and FTIR spectroscopy. Samples kept at 0 degreesC for short time rapidly crystallize into the helical form I at room temperature, whereas longer permanence times at 0 degreesC increase the stability of the trans-planar mesomorphic phase which remains stable and inhibits the normal crystallization of the sample into the helical form at room temperature. The stereoregularity of the polymer sample strongly influences the rate of formation of the mesomorphic form at 0 degreesC, Higher the syndiotacticity, easier the formation of trans-planar mesomorphic form. Very short permanence times (few hours) at 0 degreesC are enough to form and stabilize the mesomorphic form for highly syndiotactic samples, whereas for stereo-irregular samples the amount of mesomorphic form observed when the sample is removed from the 0 degreesC bath and heated to room temperature, remains always very low, even for very long permanence times (months) at 0 degreesC and the crystallization into the normal helical form is not inhibited.