The dynamic properties of semidilute solutions of a styrene-isoprene diblock copolymer in 1,1-diphenylethylene, a solvent isorefractive with the polystyrene block, have been studied by quasielastic light scattering spectroscopy. Decay time distributions having a complex structure with three separate bands have been observed at higher copolymer concentrations. The slowest mode has been attributed to diffusion of copolymer aggregates or associates. The two faster dynamic modes have been interpreted on the basis of the mean-field theory developed by Benmouna et al.