The investigation of deep levels of argon-implanted LEC-grown semi-insulating GaAs with implantation dosages ranging from 1*1011 to 1*1015 cm-2 has been performed. Using a photoinduced transient-current spectroscopy (PITCS) it was demonstrated that, for implantation dosages below 1*1013 cm-2, a negative peak or negative transient current (NTC) was observed in the temperature range from 330 to 350 K.