Materials performance in structural applications is often restricted by a transition from ductile response to brittle fracture with decreasing temperature. This transition is currently viewed as being controlled either by dislocation mobility or by the nucleation of dislocations, Fracture experiments on tungsten single crystals reported here provide evidence for the importance of dislocation nucleation for the fracture toughness in the semibrittle regime. However, it is shown that the transition itself, in general, is controlled by dislocation mobility rather than by nucleation.