Critical conditions for shear-induced structure formation in semidilute polystyrene solutions with dioctyl phthalate as a solvent are studied by light scattering and rheoiogical measurement. The critical shear rate (gamma )over dot(ex) for the onset of the nonlinear enhancement (defined in the text) of the concentration fluctuations are investigated as functions of:temperature and concentration. (gamma )over dot(cx) increases with increasing temperature or decreasing concentration, reflecting the temperature and concentration dependence of the thermodynamic and viscoelastic properties. This temperature and concentration dependence of (gamma )over dot(cx) was found to be in parallel to that of the critical shear rate for the onset of the linear enhancement (also defined in the text) of the concentration fluctuations, which can be explained in terms of a linear theory proposed by,Onuki. The critical shear stress and the critical first normal stress difference corresponding to (gamma )over dot(cx) are also explored,the physical interpretation of which will be discussed in detail in conjunction with the stress components arising from anisotropic concentration fluctuations, based on the calculation by van Egmond.