Defects in GaAs crystals were studied by internal friction. Measurements (at 10 kc/sec) show a damping peak at 140°C, corresponding to a single activation energy of 0.7 eV. The peak occurs for crystals stressed in both  and  directions. The fact that the peak height can be varied by treatments in arsenic vapor, but is not related to grown-in impurities, suggests that an intrinsic type of defect causes the peak. A nearest-neighbor pair of Ga vacancies is the simplest defect which can explain the observed peak. The possibility that the damping peak is due to isolated Ga vacancies, or to defects occupying neighboring Ga and As sites, is ruled out. © 1966 The American Institute of Physics.