A strong NDM (negative differential mobility) in n-InSb at low temperatures is predicted from a single non-parabolic band model. Calculations allowing for the anisotropy of the distribution function have been made using (1) a drifted Maxwellian, and (2) a "two-temperature" model. The calculated NDM threshold field of 550 V/cm is in an observable field range in p-n junctions. In bulk samples, where breakdown occurs at E = 200 V/cm, domain nucleation may take place at high-field inhomogeneities and contribute to the dynamics of the breakdown process and attendant microwave emission.