Dynamics of a random copolymer, poly(styrene-co-acrylonitrile), in dilute and semidilute solutions was studied by using dynamic light scattering. The solvent used was a mixture of dioxane and acetonitrile (ACN). The mixing ratio and the temperature were changed to provide the copolymer with various solvent conditions that range from a good to a Theta solvent. At low ACN contents, the autocorrelation function of the light scattering intensity had a single relaxation mode. Its diffusion changed from the mutual diffusion to the cooperative diffusion of entangled chains in the good solvent as the concentration c increased. At high ACN contents (near Theta), the diffusion coefficient decreased to 1/3 of the dilute solution value and the scattering intensity increased almost linearly with c, before the diffusion coefficient turned to increase as similar toc and the scattering intensity to decrease as similar toc(-1), indicating the cooperative diffusion of entangled Theta chains. In this high concentration range, the autocorrelation function exhibited a slow diffusion mode as well. A high friction coefficient and an extended near-linear range in the scattering intensity, especially at low temperatures, suggest dynamic clustering of chains upon increasing the concentration.