The kinetics of alpha (monoclinic) and beta (hexagonal) transcrystallization of isotactic polypropylene on aramid Kevlar 149 fibres, glass fibres and high modulus carbon fibres was investigated under isothermal and gradient cooling conditions. No difference was found between growth rates of bulk spherulites and transcrystalline layers, and Hoffman's theory led to the same results in both cases. Regarding oc transcrystallization, a transition between regimes II and III occurred near 137 degrees C and the ratio of the slopes of the two regimes was close to the theoretical value of 2. Regarding beta transcrystallization, only regime II was exhibited in the temperature range studied. However, the induction time for transcrystallization was strongly influenced by the type of fibre, which in turn-based on Ishida' s approach-resulted in variations in free energy differences at the fibre-crystallite interface for various fibres and bulk polypropylene. The respective values were 1.3, 1.5 and 2.1 X 10(-3) J m(-2) for Kevlar 149 fibres, high modulus carbon fibres than in polypropylene, showing that or crystallization is more likely to occur in Kevlar 149 fibres and high modulus carbon fibres and bulk polypropylene. Gradient-thermal measurements were performed for or transcrystallinity which allowed estimation of the activation energy of transcrystallization for the different composites. Activation energies of transcrystallinity promoted on Kevlar 149 and high modulus carbon fibres were found higher than the activation energy for bulk crystallization.