The experimental variation of the diffusion coefficient D with Zn concentration Cs has been determined at 1000, 900, 800, and 700°C from radioactive 65Zn diffusion profiles by a Boltzmann-Matano analysis. With interstitial Zn as the dominant diffusing species and its concentration controlled by the interstitial-substitutional equilibrium in which the singly ionized interstitial donor reacts with a neutral Ga vacancy to form a singly ionized substitutional acceptor and two holes, the effective diffusion coefficient is described by D=D*Cs2γp2[1+(Cs2γp)(dγpdCs)], where γp is the hole activity coefficient. The term D* equals 2DiK1pAs414, where Di is the interstitial diffusion coefficient, K1 the reaction equilibrium constant, and pAs4 the As4 pressure. The relationship between γp and the Fermi level Ef is given by γp=(Ap)exp(EfkT), where A is a constant dependent only on temperature and p is the hole concentration. This derivation for D has extended previous analyses to include both the built-in field and the nonideal behavior of holes which occurs when the impurity level broadens into an impurity band and merges with the valence band to form impurity-band tails at high Zn concentrations. The observed nonmonotonic dependence of the Zn diffusion coefficient on its concentration is a consequence of the nonideal behavior of holes at high concentrations. Quantitative comparison of D with the experimental concentration dependence has permitted the determination of γp and Ef as functions of the hole concentration.