Dynamic and electrophoretic light scattering were used to study the diffusion and electrophoretic mobility of poly(dimethyldiallylammonium chloride) as a function of polymer molecular weight in salt-free solutions. Two relaxation modes characterized as fast diffusion (D-f) and slow diffusion (D-s) were obtained from dynamic light scattering. Although the slow diffusion coefficient D-s strongly depends on molecular weight (M(w)), the fast diffusion coefficient D-f was found to be independent of M(w) over the range in the study. The fast diffusion was considered as the diffusion of a part of the polymer chain; the slow diffusion was interpreted by multichain diffusion. Electrophoretic light scattering results in the salt-free solution show that the electrophoretic mobility of the polymer is independent of M(w).