The melt spinning of metallocene catalyzed isotactic polypropylene resins was investigated. The details are presented for on-line studies performed on six miPP resins with melt flow rates (MFRs) between 10 and 100 and a Ziegler-Natta catalyzed isotactic polypropylene resin with a MFR of 35 for comparison. The on-line studies indicated that, as the molecular weight and polydispersity increased, crystallization occurred closer to the spinneret at higher crystallization temperatures and under lower spin line stresses. Further, as the spinning speed increased, crystallization occurred closer to the spinneret at higher crystallization temperatures because of increased stress in the spin line. These observations were interpreted in terms of an increased rate of crystallization caused by increased molecular orientation in the spin line with increasing molecular weight and increasing spinning speed. This "stress-enhanced" crystallization was further interpreted in terms of an increased rate of crystal nucleation. It was further concluded that the narrower molecular weight distribution of metallocene resins was the primary factor that produced differences in the structure and properties of fibers spun from these resins compared to those of Ziegler-Natta catalyzed resins of similar weight-average molecular weight or MFR.