Gallium phosphide was prepared using a horizontal refining technique. The liquid zone was maintained close to the stoichiometric composition. An ingot, doped with silicon, was zone refined. From electrical, optical transmission, and spectrochemical measurements, it is demonstrated that silicon (at concentrations in the range of 1018 at./cc) is a donor impurity with an activation energy of 0.06 ev, that silicon substitutes for gallium, and that the segregation coefficient for silicon in is 0.6.