The mode-I-plane-strain fracture toughness at initiation of various engineering plastics was determined for test speeds between 10(-4) m s-1 and 1 0 m s-1, using a high-speed, servohydraulic testing apparatus. At high rates of testing, the transient acceleration of the specimen was reduced by the use of a damping technique aimed at overcoming dynamic effects. The results obtained are correlated with fractographic analysis performed by scanning electron microscopy (SEM) and by digital image analysis of macroscopic fracture surfaces. The results show that for some materials the high values of K(Ic) measured at low testing rates are associated with deformation detectable by either method. The extent of such deformation tends to be reduced as the testing velocity is increased and the fracture toughness drops.