Differential scanning calorimetry and hot-stage optical microscopy were used to study the isothermal crystallization kinetics of unreinforced poly(phenylene sulfide) (PPS) and PPS reinforced with aramid, carbon, and glass fibers. The influence that fibers have on the crystallization kinetics of PPS was found to depend on the characteristics of the fiber as well as the type of PPS used. For one kind of PPS, fibers enhanced the crystallization rate, while for another type of PPS, reinforcing fibers had a moderate depressing effect on the polymer crystallization rate. To clarify these effects, we used a new method of quantifying the nucleation process in fiber-reinforced composites that employs a 3-D computer simulation of spherulitic crystallization. Using this method, the nucleation density in the bulk polymer, N-b, and the nucleation density on fiber surfaces, N-f, were calculated for PPS composites as a function of crystallization temperature. The calculated values of N-b and N-f were used to explain differences in the effectiveness of the fibers as well as differences in the nucleating characteristics of the two polymers.