ABSTRACT Many experiments have shown that when a frog is disturbed it escapes towards the light, and that blue light is more effective than any other colour in guiding this response (see Pearse, 1910, for a review). A previously reported experiment (Muntz, 1962 b) has shown that the greater effectiveness of blue light is not simply due to the animals’ possessing a greater sensitivity to light in this part of the spectrum, but involves colour vision; the animals will respond preferentially to blue irrespective of the intensity or saturation of the stimulating lights. They will, for example, choose a blue illuminated window in preference to a window illuminated by an additive mixture of the same blue light together with a green light, in spite of the fact that the latter window contains as much blue as the former, as well as the green.