We propose a new model for the brittle-to-ductile transition based on the Kosterlitz-Thouless concept of dislocation screening. In this model, thermal fluctuations assisted by the applied stress drive the spontaneous generation of dislocations and the instability occurs well below the melting temperature. In the limit of zero stress, our model reduces to the Kosterlitz-Thouless theory of the melting transition, and, in the opposite limit of zero temperature, we obtain the Rice-Thomson result for the brittle-to-ductile transition.