Dynamic light scattering measurements have been performed on four symmetric diblock copolymer melts, as a function of temperature, both above and below the order-disorder transition (ODT). The materials were poly(ethylenepropylene)-poly(ethylethylene) (PEP-PEE) with M(w) = 5.0 x 10(4), poly(vinylcyclohexane) (PVCH)-PEE with M(w) = 5.3 x 10(4), PVCH-polyethylene (PE) with M(w) = 1.6 x 10(4), and PE-PEE with M(w) = 2.7 x 10(4). Up to four relaxation modes were resolved. In all cases, the correlation functions exhibited a very strong, very slow diffusive mode, similar to that previously observed in polymeric and small-molecule glass formers and attributed to long-range density fluctuations. Via the Kawasaki-Stokes-Einstein relation, a correlation length or cluster size, xi was associated with this process. Above the ODT, xi was on the order of 100 nm and independent of temperature. However, below the ODT, xi apparently increased by at least 2 orders of magnitude for PVCH-PE and PE-PEE, while remaining nearly independent of temperature for the other two copolymers. For PEP-PEE, the angle dependence of the scattered intensity also reflected a correlation length of 100 nm. The cluster mode was subtracted from the correlation functions, and the residual decays were reanalyzed by Laplace inversion. Two other modes, the diffusive heterogeneity mode and the structural internal mode, were then resolved, in accordance with theory. For the PEP-PEE sample, the heterogeneity diffusion coefficient was in quantitative agreement with the self-diffusion coefficient as measured by forced Rayleigh scattering, and at least in the disordered state, the relaxation time of the internal mode was in quantitative agreement with the longest relaxation time determined by rheological measurements. For this sample, a fourth, diffusive mode was also apparent, with a time scale intermediate between the longest relaxation time and the translational diffusion of the chains. The temperature dependence of this mode was weaker than that of the viscosity or the chain diffusion, and its specific origin is unclear; however, it appears to be correlated with the frequency at which time-temperature superposition breaks down in the rheological properties.