This paper aims to study, for the first time, the effect of crystal structure on the cell formation in an isotactic polypropylene (iPP) during a solid-state foaming process using supercritical carbon dioxide (scCO(2)) as a foaming agent. Results show that the spherulite structure exerted a significant impact on the cell morphology of foamed iPP. Very interestingly, under a relatively low pressure, microcells could appear at the centers of spherulites of iPP where the melting started proceeding first. They also appeared in the amorphous domains located in between spherulites and the interlamellar regions of spherulites of iPP. The larger the size of an amorphous area, the lower the CO2 saturation pressure needed to induce cell formation. When microcells were generated in the interlamellar regions, tie fibrils bridging lamellae could be stretched. gamma-Crystals were formed at very high CO2 saturation pressure. (C) 2008 Elsevier B.V. All rights reserved.