Field strengths at which optical damage is initiated in NaCl have been measured with a mode‐locked Nd: YAG laser with pulse durations of 15 and 300 psec. Comparison with previously reported data with aQ‐switched laser shows that the field strength required for intrinsic optical damage increases by almost one order of magnitude from 106V/cm at 10−8sec to over 107V/cm at 1.5 × 10−11sec. This is in qualitative agreement with published estimates based on the electron avalanche breakdown mechanism.