We studied the effects of a step-up shear flow from zero shear rate to the given shear rate,, on formation of shear-induced structures for a semidilute polystyrene (PS)/diethyl malonate (DEM) solution below its cloud point temperature where the solution undergoes phase separation via spinodal decomposition (SD) in quiescent state. We elucidated that the effects of step-up shear can be divided into two regions: below and above a critical shear rate, gamma(c,SD). At gamma < gamma(c,SD), growing phase-separated domains via SD are found to be deformed under the flow, so that FFT spectra of the shear-microscopy images become elliptical with the wave number q(mx) at the maximum intensity parallel to the flow being smaller than the corresponding wave number q(mz) parallel to the neutral axis. However, strikingly enough, the aspect ratio q(mz)/q(mx) of the elliptic spinodal ring observed for this system was much smaller than that observed for binary fluids. The unique feature was proposed to be the elastic effect inherent in this system. When is larger than,,SD, however, initially phase-separating structures via SD are strongly deformed and distorted. Interestingly enough, the light scattering pattern was transformed from the isotropic ring pattern into the butterfly pattern. This is interpreted as follows: when gamma > gamma(c,SD), there may not be enough time for the domains composed of elastically deformed swollen-network chains to relax, and consequently the domains are cooperatively disrupted. The disrupted domains tend to squeeze solvent in order to release the elastic free energy stored in the deformed swollen-network chains, resulting in anisotropic domain more extended to neutral axis than flow direction and hence giving rise to the butterfly pattern. (c) 2006 Elsevier Ltd. All rights reserved.