Structure development during drawing was studied for three sets of polyamide-66 (PA66) fibers with density, optical microscopy, wide-angle X-ray diffraction, and Fourier transform infrared spectroscopy. The crystallinity, estimated by density measurements, remained virtually constant with increasing draw ratios, indicating that stress-induced crystallization did not occur for the PA66 fibers drawn at room temperature, but there was a rapid transformation from a hedrite morphology to a fibrillar one. The absence of stress-induced crystallization differed from the behavior of polyamide-6, and this was attributed to the stronger hydrogen bonding between polyamide chains and the higher glass-transition temperature of PA66. Polarized infrared spectroscopy was used to measure the transition-moment angles of the vibrations at 936 and 906 cm(-1), which were found to be 48 and 60degrees, respectively. The crystalline orientation was estimated from the band at 936 cm(-1) and the increase with an increasing draw ratio was in close quantitative agreement with X-ray diffraction data; this showed that infrared spectroscopy could be used reliably to measure the crystalline orientation of PA66 fibers. Because we were unable to obtain the transition-moment angle of the amorphous bands, the amorphous orientation was obtained with Stein's equation. The amorphous orientation developed more slowly than the crystalline orientation, which is typical behavior for flexible-chain polymers.