In the Alga Acetabularia intense light flashes produce transient depolarizing potentials which can be recorded intracellularly with 3 molar KCl glassmicroelectrodes. This rapid photoelectric effect has a latency of not more than 40 µsec. Spectral measurement showed a maximal sensitivity near 540 mµ. There is a second depolarization in the dark period after the flash. Lowering the temperature increases the amplitude of the fast depolarization and decreases the amplitude of the dark depolarization. Presumably the pigment and generator of the fast light-induced potential are associated with the cell membrane.