Photoexcitation of the intermediates of the bacteriorhodopsin photocycle provides valuable information on their particular rôles. Photoelectric responses from the K, M and N forms are available but the electric response of the excited O intermediate is poorly known. Double flash experiments with varying time delays between the flashes have been carried out, revealing that the photoreactions of the O intermediates in wild-type bacteriorhodopsin and in mutant E204Q are quite different. The photoreaction of the O intermediate of the wild-type bacteriorhodopsin is more-or-less similar to that of the bR-state, i.e. it translocates charge, whereas in the mutant E204Q the photoreaction is merely a small disturbance involving minor internal charge motions.