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It was found in previous study (Okumura, 1963) that a sudden change in direction of ciliary beating was evoked by flashed light in dark-adapted Paramecium. Excitability to light was affected by the relative difference in intensity of the stimulating light and background illumination. The maximum value in the spectrum sensitivity curve of Paramecium was between 330 and 350 mp. Moreover, "off-response " was evoked by ordinary light or weak ultraviolet rays, and "on-response " was evoked by strong ultraviolet rays. According to these results, it was suggested that the response to light in Paramecium was elicited by a change in membrane permeability due to the change in the molecular structure of cell mem-brane which had absorbed radiation energy from the stimulating light. In order to analyze the response to light in Paramecium, electrical properties of the cell membrane were examined during excitation by means of a microelectrode. Material and Methods All of the experiments were made with specimens of Paramecium caudatum cultivated in a vegetable powder infusion. In preparation for the experiments, the animals were washe