Isotactic polypropylene (iPP) rich in beta crystal niodification was deformed by plane-strain compression at T = 55-100 degrees C. The evolution of phase structure, morphology, and orientation were studied by DSC, X-Ray, and SEM. The most important deformation mechanisms found were interlamellar slip operating in the amorphous layers, resulting in numerous fine deformation bands and the crystallographic slip systems, including the (110)[001](beta) chain slip and (110)[1 (1) over bar0](beta) transverse slip. Shear within deformation bands leads to beta-->alpha solid state phase transformation in contrast to beta-->smectic transformation observed at room temperature. Newly formed alpha crystallites deform with an advancing strain by crystallographic slip mechanism, primarily the (010)[001](alpha) chain slip. As a result of deformation and phase transformation within deformation bands beta lamellae are locally destroyed and fragmented into smaller crystals. Deformation to high strains, above e = 1, brings further heavy fragmentation of lamellae, followed by fast rotation of crystallites with chain axis towards the direction of flow FD. This process, together with still active crystallographic slip, leads to the final texture with molecular axis of both crystalline beta and alpha phase oriented along FD. (C) 2007 Wiley Periodicals, Inc.