Evidence for defect centers having an ionization energy atEv+0.10 eV in melt‐grown GaAs is given based on: (1) Hall‐effect measurements on Cu‐diffused crystals. (2) Residual64Cu left in GaAs after extraction by indium. (3) Decrease in electron concentration of donor‐doped crystals after annealing. The failure to observe the defect level in photoluminescence is discussed. The results suggest that Ga vacancies are present in clusters in melt‐grown GaAs crystals.