At intermediate K values a region where the crack speed is constant, that is, independent of K, is observed in detergent cracking of low density polyethylene. This region is terminated at high K by one of two processes: in thin specimens the crack arrests, and in thick specimens it branches. The mechanism of crack branching involves the crack front twisting and then the initiation of a new crack at the centre of the specimen. A model has been proposed to explain this mechanism and also the fact that crack arrest or branching occur at approximately the same K value. After branching the cracks continue to propagate at constant speed along paths where K| ≈ 0 and if the specimen is wide enough, can branch again. © 1986 Chapman and Hall Ltd.