The dynamic critical behavior of ternary mixtures of polystyrene (M(w) = 3.55 x 10(5))/poly(methylmethacrylate) (M(w) = 3.27 x 10(5))/deuterated (d6-) benzene was investigated by quasielastic light scattering, pulsed-field-gradient nuclear magnetic resonance (PFG-NMR) and shear viscosity measurements. The total polymer concentration c was fixed at c is-approximately-equal-to 12.9 . c* (c* is the overlap concentration), where polymer chains were weakly entangled. The relaxation rate GAMMA(I) of polymer composition fluctuations was separated to the critical part GAMMA[c] and the background part GAMMA(b) GAMMA(c) was in good agreement with the Kawasaki function modified by Siggia, showing that the hydrodynamic backflow dominated over the diffusion as in binary fluids of small molecules. The experimental mode-coupled region was much wider than the recent theoretical prediction by Roby and Joanny. The self-diffusion of constitutional polymers did not show any influence by critical fluctuations in the present experimental time and length scales.