The technique of reflection electron energy-loss spectroscopy (REELS) has been applied to the electrons channeled along crystal surfaces of GaAs under the surface resonance conditions of reflection high-energy electron diffraction (RHEED) when the crystal is terminated so that some transmitted electrons are observed. Observations of REELS spectra and the splitting of Kikuchi lines are consistent with the results of the dynamical diffraction calculations of the electron wavefields in the crystal.