We　present　the　study　of　the　ultrafast　carrier　dynamics　in　bulk　GaAs　at　6　K,　by　time-resolved　optical　reflectivity　in　a　wide　range　of　excitation　densities.　We　observe　an　exciton-phonon　interaction　at　low　excitation　densities,　in　terms　of　a　mono-exponential　relaxation.　Further　increase　of　excitation　density　above　a　critical　value　n(c),　stimulates　a　bi-exponential　relaxation　in　which　the　fast　one　is　added　owing　to　an　exciton-exciton　scattering　channel.　Our　results　reveal　n(c)　similar　to　2　x　10(24)　photons/m(3),　about　an　order　of　magnitude　higher　than　the　Mott　density　previously　determined　by　photoluminescence　and　terahertz　spectroscopy　measurements　and　theoretical　calculations.