The tensile and fracture behaviour of neat alpha and beta nucleated isotactic polypropylene and rubber-modified alpha and beta nucleated isotactic polypropylene has been investigated at test speeds of 0.0001-10 ms(-1) in the temperature range -30 to +60degreesC. The presence of the beta phase had little effect at low temperature. However, at +25 and +60degreesC, it increased the speeds corresponding to the ductile-brittle transition in the neat polymer by more than three decades. This behaviour has been linked to changes in microdeformation mechanisms observed at the lamellar and spherulitic level, an increase in cavitational deformation in tensile tests and an increase in the strength of the beta relaxation in dynamic mechanical spectra. In the blends, the presence of the beta phase led to somewhat higher energy dissipation in regimes of ductile fracture. However, the ductile-brittle transitions were not significantly affected. The modifier phase was therefore infer-red to control the initiation and propagation of the plastic zone ahead of the crack tip during fracture.