Excitation of bacteriorhodopsin (bR) at an electrode-electrolyte interface generates transient photocurrents as evidenced by the turning of an incident light on and off. By use of a gold electrode as substrate, on which an oxide layer can be formed in a controlled manner, we have found two types of photocurrents, both originating in the excitation of bR. For the first type, arising most probably from the pH response of the surface oxide layer due to proton release/uptake by bR, the magnitude of photocurrent well paralleled the amount of surface oxide up to about one monolayer of Au2O3 and reached a maximum roughly equal to those observed on SnO2 electrodes. The second type of photocurrent, being 3- to 4-fold smaller than the first one and essentially potential independent, arises presumably from simple charging, again through proton release/uptake by bR, of the electric double layer at the interface.