Mechanical properties of melt-spun syndiotactic polypropylene single filament fibers were studied in relation to their formation conditions. Depending on the processing parameters, fibers with rubber-elastic properties can be prepared. Dynamic mechanical studies show almost unchanged rubber-elastic properties after 1000 test cycles. A model of the fiber morphology is introduced to explain the rubber-elastic behavior: small micel-like crystals containing molecules in the planar all-trans conformation act as physical crosslinks in the amorphous matrix.