The self-diffusion of Cd in CdS has been measured under a variety of doping and firing conditions. Under saturated Cd pressure the diffusion coefficient is given by $D=3\mathrm{exp}(\ensuremath{-}2.0 \mathrm{eV}/kT)$. Under sulfur pressure at 800\ifmmode^\circ\else\textdegree\fi{}C the diffusion coefficient is found to be linearly dependent on the donor-impurity concentration. A new mechanism for diffusion is proposed to explain the results. No evidence for interstitial Cd was observed and it is suggested that Cd and sulfur vacancies (Schottky defects) are the dominant native defects in CdS.