Elongational flow behavior of a polystyrene-block-poly(ethylene butylene)-block-polystyrene (SEBS) triblock copolymer melt with cylindrical morphology is investigated by elongational Row opto-rheometry (EFOR), transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS). The SEES film is first roll-processed to align the PS cylinders in a preferred direction. The uniaxial elongation is applied either parallel (denote Case I) or perpendicular (Case II) to the cylinder axis. Transient tensile stress sigma((epsilon) over dot(0); t) and birefringence Deltan((epsilon) over dot(0); t) are measured with a constant Hencky strain rate (epsilon) over dot(0) ranging from 0.01 to 1.0 s(-1) at various temperatures between PS glass transition, T-g(PS) and the order-disorder transition, T-ODT, of the SEBS. The data suggest that either the PS or FEB domains is preferentially elongated in the early stage of elongation, depending on the initial alignment of the cylinder phase. On further elongation, the elongational viscosity of the Case I melt exhibits strain-induced softening behavior in the final stage of elongation, whereas that of the Case II melts clearly displays strain-induced hardening behavior. The TEM and SAXS data of the samples elongated with a (epsilon) over dot(0) = 1.0 s(-1) show that the cylinders are mostly inclined approximately by 40-50 degrees to the direction of elongation, whereas they are mostly aligned, parallel to the elongation direction on slower elongation. The morphology of highly elongated SEES melts is governed by the applied strain rate and temperature, regardless of the initial orientation of the cylinders. (C) 2001 Elsevier Science Ltd. All rights reserved.