The deformation behaviour and the fracture resistance of a range of nylon 6,6/ABS alloys of varying composition both with and without the presence of glass fibres were investigated. The deformation behaviour was characterized by careful measurements of the volumetric strain during tensile tests in order to understand the relative roles of cavitation and shear yielding in these materials. The fracture resistance was investigated in detail in the fracture mechanics sense by characterizing the J-integral fracture initiation toughness. In materials exhibiting stable crack growth, a new parameter, namely, the plateau value of the J-integral fracture resistance curve, was measured directly and represented the resistance of the material to stable crack growth. The results showed that the relationship between the deformation behaviour and fracture resistance was related to the extent of damage that developed in the crack-tip zone. Substantial additional toughening was developed during the crack extension stage both in the presence a nd absence of glass fibres. Glass fibres were found to promote shear yielding and, as a result, enhance both the fracture initiation as well as the fracture propagation resistance of the nylon 6,6/ABS alloys.