We have performed a Monte Carlo simulation of high‐field transport in GaAs using a realistic band structure obtained by the empirical pseudopotential method. On this basis, a detailed study of the band structure dependence of impact ionization in GaAs is given. Our method avoids the use of the effective mass theorem or the Kane model of nonparabolicity, which are no longer accurate at high electron energies. We show (i) that the orientation dependence of the impact ionization rate is negligibly small, (ii) that the saturation velocity of electrons in GaAs is close to 6×106cm/s at extremely high fields (this value is determined to a large extent by the band structure, and (iii) that the previous theories of impact ionization as given by Wolff, Shockley, and Baraff have numerous limitations.