The yield strain has been measured over a wide range of temperatures for four polyethylene grades which differ with respect to both their short chain branch content and their molecular weight. The results have shown that all the materials exhibit a transition in their mechanical behaviour at sub-ambient temperatures. The transition temperature is shown to increase as the crystallinity is increased. It is proposed that this transition temperature is related to an interlamellar sheer process. The yield strain measured over a range of temperatures generally increases as the crystallinity is reduced.