The effect of polydispersity on the formation of flow-induced, oriented morphology in polyolefins is investigated by polarized light imaging and small angle X-ray scattering. A torsional shear flow was applied at different temperatures to model polyethylene blends (bimodal and trimodal hydrogenated polybutadienes) comprising of two kinds of long chains with different molecular weight (1080 and 1770 kDa) in a matrix of short chains (18 KDa), and the results were compared to those of polydisperse materials. While a single boundary associated with the threshold flow conditions for the onset of oriented morphology is observed in the bimodal blends, two boundaries corresponding to the orientation of the longest chains (1770 kDa) and next longest chains (1080 kDa) are detected in the trimodal blends. The results obtained, herein, are extended by inference to polydisperse polymers. It is demonstrated that the shear rate dependence of the critical specific work parameter for the onset of oriented morphology in polydisperse polymers is dictated by the molecular weight distribution and that the longest chains mainly control the process with some contribution from shorter chains involved in the formation of flow-induced precursors at higher flow rates. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 621-628, 2011