The drift velocity as a function of the electric field in gallium arsenide was measured directly by studying the response of a reverse‐biased Schottky‐barrieri‐n+GaAs photodetector to a step input of light. The photoresponse was dominated by trapping of electrons in theiregion. Analysis based on a simple model yielded the drift velocity—electric field characteristic in GaAs up to fields of 9000 V/cm. A region of negative differential mobility was observed to exist between a peak field of 2200 V/cm and an apparent valley field of 8000 V/cm.