The velocity-field characteristics in zinc-blende GaN are calculated from the Boltzmann equation, using realistic energy bands taken fromab initiotheory. The drift velocity and the high-field negative differential resistance are shown to be largely determined by the inflection point in the bands centered around the &Ggr; valley, instead of the usual intervalley scattering. We analyze the relative importance of these competing mechanisms for GaN andAl0.5Ga0.5N.The importance of this anomaly to device properties is also discussed. ©1997 American Institute of Physics.