Solutions of a nearly symmetric polystyrene-polyisoprene diblock copolymer in toluene, a neutral solvent, have been examined by dynamic light scattering, at compositions both above and below the order-disorder transition. In all cases the scattered intensity autocorrelation function exhibits two distinct modes. The faster mode is diffusive and decreases in rate approximately exponentially with increasing concentration. The magnitude of the diffusivity suggests that it reflects the translational diffusion of the block copolymers. The slower mode scales with approximately the third power of the scattering vector and is tentatively attributed to cooperative rearrangements of microdomains. Furthermore, the slower mode undergoes a sharp decrease in rate over a narrow range of composition; this range of composition also corresponds to the best estimate for the location of the order-disorder transition.