Dynamic light scattering measurements on a symmetric polystyrene-polyisoprene diblock copolymer, M(w) = 3.4 x 10(5), dissolved in neutral good solvents are reported. The solvents comprise mixtures of toluene and alpha-chloronaphthalene, and the solvent compositions necessary to index-match the entire copolymer, the polystyrene block, and the polyisoprene block have been determined. The first of these, the zero average contrast (ZAG) condition, is particularly advantageous as it eliminates the cooperative diffusion mode, thus enhancing the relative amplitude of the elusive internal mode. However, the ZAC condition is sensitive to small changes in either solvent composition or temperature. In dilute and semidilute ZAC solutions, the correlation functions consistently show two modes. The slower mode is diffusive, with a diffusivity in close agreement with that measured by pulsed-field-gradient NMR; it is thus interpreted as the heterogeneity mode, which is visible due to chain-to-chain variations in composition. The faster mode is the internal mode, with a decay rate that is independent of scattering angle and decreasing with concentration; the concentration dependence is that observed for the viscoelastic longest relaxation time. For polymer concentrations within a factor of 2 of the order-disorder transition, the total scattered intensity increases markedly, which is attributed to the appearance of large-amplitude concentration fluctuations. It is also shown that in dilute solution, the heterogeneity and cooperative modes have comparable decay rates, which can complicate the interpretation of the diffusional second virial coefficient, k(d), for copolymers.