Three poly(ethylene-1-hexene) copolymers (which are characterised by different degrees of branching) were cast as flat film discs from a decalin solution at various initial dilutions. Structural analyses of these materials have shown that the stem length (the length traversed by a polymer chain within the crystal lamellae) for each grade is varied widely by altering the solutions initial dilution. Tensile yield stress measurements have been performed on these cast films in the temperature range -80 to +80 degreesC, a temperature range which includes the beta -relaxation temperature for these materials. The results have shown that, above T-beta, the stem length dependence of the yield stress is generally in agreement with the modified crystal plasticity approach previously used successfully to model the yield behaviour of a range of bulk cast polyethylenes. However, the results have also shown quite clearly, that in terms of morphology, it is not just the stem length which determines the yield stress but there is also a contribution due to the morphology of the lamellar fold surface. It is thought that the contribution of the fold surface to the macroscopic yield stress is attributable either to the energy to nucleate a screw dislocation within the crystalline lamellae or to the inter-lamellar shearing process. (C) 2001 Elsevier Science Ltd. All rights reserved.