Irradiation of GaP with 2 MeV electrons leads to the linear growth of infrared absorption lines at 424 and 419 cm-1 with a strength ratio of 1.45+or-0.15. These lines were previously attributed to the vibrations of a displaced phosphorus atom in a site of axial symmetry but it is now shown that they can be explained by a single mode of vibration 69Ga and 71Ga respectively. Low-temperature irradiation confirm the authors' earlier data (1971) that the lines do not arise from a primary defect and imply the mobility of displaced interstitial atoms. A comparison is made with the observations of interstitial trapping by substitutional boron impurities.