This paper describes a refined approach to the problems of identification and comparrison of synthetic fibers encountered in forensic science case work. The necessary apparatus consists of a polarizing microscope equipped with a Mettler hot stage. Observations concerning the variation of a fiber's birefringence with temperature have been found to be very useful, both for determining the generic type on an unknown fiber and for detecting subtle differences among fibers of the same generic type from different sources. Data, including semilogarithmic plots of birefringence as a function of temperature, are given for nine different generic types of synthetic fibers.