We have investigated relaxations in a semidilute solution of a high molecular weight polystyrene in the vicinity of Theta conditions, using three different techniques : dynamic light scattering, relaxation of flow after cessation of shear recorded through the decay of the intensity of scattered light, and flow relaxation after cessation of shear recorded rheologically. We have used a solution at the Theta temperature of polystyrene with molecular weight M(w) = 4.9 x 10(6) in dioctyl phthalate. All three techniques yield similar values of the viscoelastic relaxation time, at various temperatures between 12 and 35 degrees C. Two additional modes are observed with dynamic light scattering : the well-established gel diffusion mode of the transient network of the entangled semidilute solution and an additional mode intermediate to the previous two. It is found that the shear field enhances only the longest viscoelastic relaxation mode.