Measurements of the Charpy notched impact strength of polydiancarbonate indicate that the polymer can exist in one of two modifications A and B. There is a temperature (below room temperature) above which the A form is unusually tough; the B modification is comparatively brittle at all temperatures. The A form can be converted to the B form by an annealing process which proceeds rapidly above 80°C. We have not been able to determine the precise morphological difference between the A and B forms but most of the data can be explained by making a hypothesis about the yield behaviour of the A form.