A single layer of foam bubbles between parallel plates fails (fractures) when subjected to an applied pressure. The fracture can occur in both a ductile mode and a brittle mode. It is shown that dissipative effects during propagation of a brittle crack lead to a spontaneous brittle-to-ductile transition, which occurs dynamically during fracture and can be experimentally resolved in its details. The transition occurs when the driving pressure at the crack tip drops below that necessary to maintain supersonic propagation of the brittle crack. A theory taking into account the lateral crack opening together with air flow in the crack successfully predicts the occurrence and location of this novel spontaneous transition. (C) 2012 The Society of Rheology. [http://dx.doi.org/10.1122/1.3687425]