We report on the first successful expression of the light driven H+ pump, bacteriorhodopsin, into the plasma membrane of oocytes from Xenopus laevis. The light induced photocurrents which reflect the pumping of H+ by BR were analysed under voltage clamp conditions. At least 100 active BR molecules per microns 2 were expressed in the plasma membrane so that both the voltage clamp and giant patch clamp method could be applied. We show that H+ pumping by BR is modulated by the membrane potential, i.e. the pump current shows strong voltage dependence in the range measured between -165 mV to +60 mV.