Unicellular thermophilic cyanobacterium Synechococcus elongatus displayed phototaxis on agar plate at 55°C. Equal-quantum action spectra for phototactic migration were determined at various fluence rates using the Okazaki Large Spectrograph as the light source. The shapes of the action spectra drastically changed depending on the fluence rate of the unilateral monochromatic irradiation: at a low fluence rate (3 μmol/m²/s), only lights in the red region had significant effect; at a medium fluence rate (10 μmol/m²/s), four major action peaks were observed at 530 nm (green), 570 nm (yellow), 640 nm (red) and 680 nm (red). At high fluence rates (30–90 μmol/m²/s), the former two peaks remained, while red peaks at 640 nm and 680 nm disappeared and, interestingly, an action peak around 700–740 nm (far-red) newly appeared. These results indicate that two or more distinct photoreceptors are involved in the phototaxis and that suitable photoreceptors are selectively active in response to the stimulus of light fluence rates. Far-red or red background lights irradiated vertically from above drastically inhibited phototaxis toward red light or far-red light, respectively. These results indicate involvement of some phytochrome(s).