A detailed study of the change in hot-electron mobility with applied electric fields in n-type InAs at 77°K is presented. Both theoretical and experimental results are presented. Electron-electron collisions are assumed to be sufficiently frequent to determine the energy and momentum distribution of the electrons. The effects of ionized-impurity scattering are included in the momentum balance equation. The nonparabolicity of the conduction band is included in the theoretical calculation. The effect of the overlap integral which results in the nonparabolicity is included in the manner of Ehrenreich by utilizing several terms of the scattering matrix element. For low electric field (<150 V/cm), a slight increase (approximately 10%) in the mobility with applied electric field is observed. For higher electric fields (>150 V/cm), a strong decrease in the mobility with applied electric field occurs. This decrease is observed up to the point of impact ionization at approximately 800 V/cm.