Acoustic emission (AE) has been coupled with slow crack propagation experiments in polystyrene to probe the mechanisms of damage and fracture. The single AE active mechanism recorded during slow crack growth is probably caused by failure of the craze fibrils in the craze zone at the propagating crack tip. The AE event rate and the AE energy release rate varied linearly with the crack speed indicating that the AE depends only upon the amount of fracture surface created independent of the crack speed. Since the duration of the AE was less than 1% of the total time of the experiment, it is concluded that crack propagation is discontinuous even at the highest crack speeds investigated.