The dynamic light scattering (DLS) properties of four symmetric polystyrene-polyisoprene diblock copolymers have been examined in the neutral good solvent toluene. Concentrations (c) ranged from the dilute, through the semidilute, and into the concentrated regimes. Pulsed-field-gradient NMR has been used to determine the translational diffusivities (D-s) of the same copolymers, at selected concentrations. The results are in substantial agreement with models developed by Benmouna and coworkers and by Semenov. Three modes are predicted : cooperative diffusion (D-C), corresponding to relaxation of fluctuations in polymer concentration; an internal mode (Gamma(I)), reflecting relative motion of the two blocks within one molecule; a heterogeneity mode (D-H), due to chain-to-chain fluctuations in composition and which relaxes by translational diffusion. In dilute solutions, a single, diffusive mode is seen and is attributed to a superposition of D-C and D-H. In semidilute and concentrated solutions, D-C and D-H are cleanly resolved. D-C shows the expected scaling with c and molecular weight (M), i.e., D-C similar to c(0.7)M(0). Comparison with the NMR results confirms that the values of the diffusion coefficients D-H and D-s are very similar; this result extends the previous work of Balsara and co-workers, who first noted the appearance of a block copolymer DLS mode resembling translational diffusion. Selected measurements in two other good solvents, THF and chloroform, establish that D-C and D-H are independent of solvent refractive index (n(s)), as expected. The relative amplitudes of the cooperative and heterogeneity modes also scale with c, M, and n(s) in agreement with theory. The predicted internal mode is generally difficult to resolve but is seen for the highest molecular weight sample examined. The concentration dependences of D-H and D-s are insensitive to the order-disorder transition, in agreement with previous measurements in solutions and melts. Preliminary measurements of D-H with a novel Fourier transform heterodyne DLS apparatus were also in good agreement with the other techniques. Finally, the results are compared with previous DLS measurements of symmetric diblock. copolymers in solution. It is proposed that the apparent discrepancies among the previous studies can be resolved, by appropriate assignment of the heterogeneity mode.