Forced Rayleigh scattering has been used to investigate tracer diffusion of a tetrahydrothiophene-indigo derivative (TTI), a photochromic dye molecule, in a macroscopically isotropic lamellar poly(styrene-b-isoprene) diblock copolymer of relative high molecular weight. As anticipated from the large difference of segmental mobility between the constituent blocks, three relaxation processes termed fast, intermediate, and slow, respectively, have been observed as a function of temperature and grid spacing for the decay curve of scattered light from the photoinduced grating in the experiments. The fast process exhibits a q(2) dependence, which is characteristic of Fickian diffusion, whereas the slow process is independent of q. Model calculations have identified the nature of these two processes. In addition, the role of the size of the grid spacing chosen in these experiments, relative to the lamellar period of the microstructure, has been elucidated. A means of extracting diffusion coefficients for TTI in PI and PS lamellae from the experimental results has been demonstrated. A comparison of the diffusion coefficients obtained from the PI and PS lamellae in the block copolymer with those of TTI diffusion in PI and PS homopolymers revealed differences in the temperature dependence between the homogeneous and microstructured materials.