The influence of annealing on the birefringence of nylon-6 fibers was examined. These fibers were found to show considerable optical anisotropy. Three independent techniques were used to study the optical anisotropy in these fibers. The first technique to determine the diameter of the fiber from the diffraction pattern was using a He-Ne laser beam. The second technique was the application of the Becke-line technique to study the effect of annealing on the skin layer of these fibers at different temperatures for a constant time of 5 h. The third is the application of a double-refracting interference microscope (the Pluta polarizing interference microscope). The double-beam technique is considered as the most suitable technique for determining the mean refractive indices and the double refraction of the annealed samples at different temperatures. (155 and 175 degrees C and at two constant times 7 and 9 h). Microinterferograms and curves are given for illustration.