It is shown that inn‐InSb at 77 K, mode I of acousto‐electric excitation exists in transverse magnetic fields above 2 kG, for whichqR≪ 1 (qis the acoustical wave number, andRis the Larmor radius) if the drift motion restores a mean free pathlto giveql> 1. Threshold curves for mode I are calculated. Allowing for the magnetic field dependence of mobility, the applicability of quantum‐mechanical and hydrodynamical theory in different field regions is discussed.