The development of the morphology and the alignment of lamellae in melt elongation of blends of an asymmetric linear styrene-butadiene block copolymer (LN3) and polystyrene (PS 158K) was investigated. PS 1.58K and LN3 formed two-phase polymer blends with PS 1.58K resp. LN3 inclusions, depending oil the concentration of polystyrene. The block copolymer was arranged in a lamellar phase with a lamellae thickness of similar to 13 nm. Our rheological experiments revealed that the complex modulus, the elongational viscosity and the recovered stretch of the blends primarily resulted from a superposition of the properties of the blend components. In melt elongation, pure LN3 started to crumple at a small Hencky strain. In the blends, the presence of the PS 158K inclusions led to a macroscopically more uniform elongation, but With all anisotropic Poisson ratio. The LN3 inclusions in the PS 158K matrix were deformed into a filament-like shape. In the blends with a LN3 matrix the alignment of the block copolymer lamellae parallel to the loading direction increased with applied extensional strain. In the latter case, the lamellae thickness did not decrease significantly. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 112: 1319-1329, 2009