Almost all major phyla of invertebrates and lower vertebrates display a direct sensitivity of their chromatophores to light by either dispersing or—in rare cases—aggregating the pigment granules within the cell. This “primary response” of color change is an accessory component of the “dermal light sense” and characterizes the chromatophores as an independent receptor for light and effector of the chromomotor response. Photosensitivity does not seem to be restricted to certain parts of the pigment cell but is rather supposed to be a ubiquitous property of the chromatophore. Experiments on partially illuminated chromatophores show that the photopigment, whose chemical composition is still unknown, is localised within the plasma membrane or the cytoplasmic ground substance. Threshold responses for a just visible reaction are much higher than for background responses and have for some pigment cells been determined to be in a range between 0.2 and 0.5 erg/sec/cm2. The physiological significance of the photosensory reaction is closely related with thermoregulation and the protection of underlying tissue against harmful radiation. The chain of events involved in photosensory transduction remains to be further studied and can at present be interpreted only on the basis of related phenomena like retinal pigment migration and the light-sensitivity of simple non-pigmented contractile systems.