The rhodopsin nature of the photoreceptor for the behavioural light responses in Chlamydomonas has originally been revealed by action spectroscopy. Meanwhile most physiological experiments and the identification of all-trans-retinal in cell extracts favour that this chlamyrhodopsin contains an all-trans-type retinal chromophore with strong similarity to the light sensors SR I and SRII from Halobacteria. Reconstitution of retinal-deficient cells with [ 3 H]retinal identified a single retinal protein with a MW of 30,000. Chlamyrhodopsin triggers a photoreceptor current in the eyespot region resulting in direction changes or phototaxis. Furthermore, when the light stimulus oversteps a critical level, two flagellar currents appear, which are the basis for photophobic responses. The physiological, electrophysiological and biochemical experiments suggest that all behavioural responses are triggered by a single rhodopsin-type receptor.