The fracture behaviour of an isotactic polypropylene and an impact modified isotactic polypropylene containing 15 wt% of ethylene-propylene rubber has been investigated at test speeds of between 0.1 mm/s and 14 m/s, using compact-tension specimens. The homopolymer displayed a ductile-brittle transition as the test speed was increased, which was associated with a transition from shear deformation to crazing. Deformation of the modified polypropylene was characterized by stable crack propagation over the whole range of test speeds, stress-whitening at the crack tip and, at lower speeds, by the formation of extensive shear lips on the fracture surfaces. An observed increase in the size of the crack tip damage zone with test speed suggested a transition from plane stress to plane strain conditions. This is discussed in terms of a simple model, which takes into account the variation of the sample stress state as the test speed increases. (C) 2000 Elsevier Science Ltd. All rights reserved.