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The effect of silica on the crystallization and melting behavior of a highly isotactic, well characterized isotactic polystyrene (i-PS) have been investigated. The origins of the various endotherms obtained upon heating have been defined by partial scanning experiments and by a study of the effect of heating rate using differential scanning calorimetry (DSC). The presence of 1 part silica in 100 parts polymer (1 pph) decreases the maximum degree of crystallinity considerably but has a minimal effect on the rate of crystallization. Analysis by the Avrami method shows that the silica does not affect the overall rate of crystallization significantly. The decrease in the crystallinity indicates that silica affects the level of secondary crystallization, thus the crystal perfection. / The surface morphologies and growth rates of i-PS spherulites, as studied by photomicroscopy, were not affected by 1 pph of silica. The experimental data were fitted to a modified form of the Hoffman-Lauritzen equation. / The effect of silica on spherulite growth rates and surface morphologies of isotactic poly(propylene oxide) (i-PPO) have also been investigated by optical microscopy. Two distinct i-PPO samples of different molecular weights were used, each of which was highly isotactic. The addition of silica has a pronounced effect on the morphology of the spherulites, producing dendritic type morphology. Upon step-crystallization, the spherulites exhibited mixed morphologies, i.e., fibrillar and ringed. Silica depresses the spherulite growth rates throughout the entire temperature range. The effects were more profound as the quantity of filler increased. The growth rate-temperature behavior was analysed in terms of the classical Hoffman-Lauritzen equation and a modified version to take into account the polymer-filler interaction.