We have measured the electron drift mobility in a-Si:H at high electric fields (E≤3.6×105 V/cm). The a-Si:H pin structure was prepared at Palaiseau, and incorporated a thick p+ layer to retard high field breakdown. The drift mobility was obtained from transient photocurrent measurements from 1 ns-1 ms following a laser pulse. Mobility increases as large as a factor of 30 were observed; at 77 K the high field mobility depended exponentially upon field (exp(E/E0), where E0 = 1.1×105 V/cm). The same field dependence was observed in the time range 10 ns-1 μs, indicating that the dispersion parameter change with field was negligible. This latter result appears to exclude hopping in the exponential conduction bandtail as the fundamental transport mechanism in a-Si:H above 77 K; alternate models are briefly discussed.