The present study was undertaken to investigate the mechanism for primary nucleation in polymer crystallization. Previous studies in this area have not been conclusive. Experiments on polypropylene with long induction times, studied by small- and wide-angle X-ray scattering (SAXS and WAXS), reveal the onset of long-range ordering prior to crystal growth. Rapid crystallizations studied by melt extrusion indicate the development of well-resolved oriented SAXS patterns associated with long-range order before the development of crystalline peaks in the WAXS region. The experimental results suggest pre-nucleation density fluctuations play an integral role in nucleation of polymer crystallization.