The quasielastic scattering function of a diblock copolymer chain in dilute solution has been evaluated according to the Rouse-Zimm theory (valid for Gaussian,chains with preaveraged hydrodynamic interactions), and also through Brownian dynamics simulations which include net repulsions between units of different; type (A or B) and fluctuating hydrodynamic interactions. Opposite contrast factors are defined for the A and B units (i.e., the solvent refractive index is assumed to be intermediate with respect to those of A and B). Both types of calculations agree in the main features : The scattering function can be expressed as a sum of exponential terms, whose relative contributions are completely different from the ones calculated from the Pecora analysis for homopolymer chains. For small or moderate values of the scattering variable, close to the peak for the total scattering intensity, a single exponential is obtained, but its exponent depends both on the translational diffusion coefficient, and on the chain first internal relaxation time.