The structure and dynamics of a strongly asymmetric poly(ethylene propylene)poly (dimethylsiloxane) (PEP-PDMS) diblock copolymer in the melt have been studied over a wide temperature range. Small-angle neutron scattering reveals that the sample exhibits two stable phases in this temperature range: Above the order-to-disorder transition temperature, it is disordered, whereas the domain structure is body-centered cubic (bcc) below, being stable down to the lowest temperatures measured. In the disordered state, dynamic light scattering (DLS) in the polarized geometry reveals the heterogeneity mode and the cluster mode. In the bcc phase, the PEP and the PDMS blocks form the inicellar cores and the matrix, respectively. Here, two modes are observed in DLS, and the diffusion coefficients measured using pulsed field gradient (PFG) NMR are broadly distributed with the most probable diffusion coefficient coinciding with the slow DLS mode. We attribute the fast process in the bcc state to concentration fluctuations of the micellar cores (PEP), relaxing by mutual diffusion of the micelles with copolymers dissolved in the PDMS matrix. The slower process in the bcc state is ascribed to activated long-range self-diffusion of single copolymers from micelle to micelle through the PDMS matrix. This assignment is corroborated by the good coincidence of the reduced diffusivities with the ones from the literature. However, this mode may also be assigned to the rearrangement of entire micelles.