The structural details of high-speed melt-spun nylon 6 fibers at spinning speeds ranging from 4500 to 6100 m/min were investigated by solid-state proton nuclear magnetic resonance (1H NMR) spectroscopy, density and birefringence measurements, differential scanning calorimetry (DSC), and X-ray diffraction (XRD). The analyses of the proton spin-lattice relaxation times in the rotating frame and correlation times confirmed the existence of three different phases, the immobile crystalline, intermediate rigid amorphous, and mobile amorphous regions, in the fiber sample. At spinning speeds lower than 5200 m/min, the portion of the crystalline phase increased at the expense of the rigid amorphous region and then reached a plateau afterward, from which the mobile amorphous portion increased. Combined analyses of density and birefringence measurements, DSC, and XRD in conjunction with NMR results indicated that the formation of the γ crystal became predominant compared to that of the α crystal. The orientation factor of the crystalline phase increased slightly with increasing spinning speed, whereas the amorphous orientation factor decreased because of the increase of the purely amorphous region. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1285–1293, 2000