We report a convenient method to obtain all‐optical light modulation in bacteriorhodopsin films using a degenerate four‐wave mixing geometry. Chemically stabilized films of bacteriorhodopsin in a polymer matrix for which the lifetime of the excitedMstate is tens of seconds were used to demonstrate all‐optical light intensity modulation. The films are observed to be stable over a period of 4 years. Due to the sensitivity of the films, small intensities of order microwatt/ cm2are required in the modulation experiments. Furthermore the fast photochemical transition fromMtoBpermit reasonably fast modulation speeds independent of the slow thermalMandBrelaxation time. The experimental system also acts as an all‐optical switch where a low power blue pulse turns on a signal red beam. ©1995 American Institute of Physics.