We present results of ensemble Monte Carlo simulations of the initial femtosecond dynamics of hot‐carrier relaxation and scattering processes in bulk GaAs. Contributions of each operative scattering mechanism to the primary stage of carrier relaxation are investigated by turning them on or off separately. We find that when scattering to bothLandXvalleys is energetically possible, the initial relaxation process, which occurs on a time scale of ∼50 fs, is dominated by the intervalley scattering out of the &Ggr; valley, but cannot be described by a single, averaged scattering time. When the carriers are excited between theXandLvalleys, thermalization occurs on a time scale of ∼140 fs, which is mainly due to &Ggr;‐Lintervalley scattering, and then, optical‐phonon ande‐escatterings start to contribute in the relaxation. At low temperatures, however, optical‐phonon scattering plays only a secondary role.