The special symmetry properties of second‐harmonic generation at the surface of a GaAs crystal are used in a technique which measures the shape of the fast pulses from a mode‐locked Nd‐glass laser. The pulses studied were found to have a full width at half power of between 4 and 6 picoseconds. The technique is capable of measuring pulse widths at least as short as 4 × 10−13sec.