Forced Rayleigh scattering (FRS) has been used to probe self-diffusion and tracer diffusion in unentangled, symmetric diblock copolymer melts. Two poly(styrene-b-2-vinylpyridine) (PS-PVP) diblocks, with M(w) = 1.1 x 10(4) and 2.2 x 10(4), were synthesized anionically and end-labeled with a photochromic o-nitrostilbene derivative, and two PS homopolymers, with M(w) = 1.4 x 10(4) and 2.5 x 10(4), were labeled with the same dye, randomly along the chain. Rheological measurements demonstrated that the lower M(w) diblock was in the disordered state and the higher M(w) diblock was in the ordered (lamellar) state, over the measurement temperature range (110-210-degrees-C). The ordered samples were quenched, in the sense that no attempt was made to induce a preferred orientation to the lamellae. The self-diffusion coefficients of the two copolymers superpose, when scaled by the ratio of molecular weights, as expected for disordered Rouse chains, thus indicating that the lamellar order has little effect on the overall mobility. Similarly, the copolymer and homopolymer self-diffusion coefficients are nearly identical, for equal total M(w). This result is interpreted as reflecting unhindered motion of the copolymer chains in the lamellar planes. Tracer diffusion measurements for the homopolymers and the lower M(w) copolymer in the ordered copolymer matrix also showed little difference from the corresponding self-diffusion coefficients but were qualitatively consistent with perturbation theory. In all cases, the FRS signals were well-described by single exponential decays, possibly suggesting that diffusion through the lamellae is not greatly retarded relative to diffusion in the lamellar planes and/or that the orientational correlation length of the lamellae is small.