Multiple-beam Fizeau fringes were used for studying the effect of the annealing process on the refractive indices and birefringence of natural silk fibers (best yellow Italian silk, Ford & Co. Ltd). Silk fibers were annealed at a constant time of 2 h with different annealing temperatures ranging from 60 to 160 +/- 1 degrees C. A scanning electron microscope was used for measuring the cross-sectional shape and a longitudinal view of the natural silk fibers. The Becke-line method was used for measuring the skin refractive indices and the birefringence of the natural silk fibers. The thermal coefficient of the refractive index, Cauchy dispersion constants, dispersive power, dielectric constant at infinity, polarizability per unit volume, isotropic polarizability, and isotropic refractive index were determined interferometrically. Microinterferograms and curves are given for illustration.