Light scattering spectroscopy was used to study the diffusion of mesoscopic optical probe particles through concentrated aqueous solutions of high molecular weight hydroxypropylcellulose. Optical probe spectra in solutions of this polymer are bimodal, consisting of fast and slow modes. Hydroxypropylcellulose solutions show an apparent viscoelastic solutionlike-meltlike transition, at which the concentration dependence of the solution viscosity eta changes from a low-concentration stretched exponential form to a high-concentration power-law form. We studied the influence of this apparent viscoelastic transition on the optical probe spectrum S(q,t). The concentration dependence of the fast mode has a sharp change near the solutionlike-meltlike transition concentration ci. The slow mode fitting exhibits smaller changes near c(+) in its concentration dependence. It has been suggested that solutionlike-meltlike transition seen in the viscosity measurements is only an artifact of the curve-fitting procedures used to analyze that data. Our results here on an independent physical quantity, namely S(q,t), confirm the physical reality of the solutionlike-meltlike transition.