THE unicellular alga Chlamydomonas reinhardtii responds to weak flashes of light by changing its swimming direction and to brighter flashes with transient backward swimming (the stop or phobic response)^1,2. In continuous light the cells swim towards or away from the light source (phototaxis)^3,4. This behaviour is controlled by a visual system with a rhodopsin as the functional photoreceptor^5,6. Physiological experiments under different ionic conditions have suggested that ionic processes are involved in the signal transduction from the photoreceptor to the flagella^1,2,4. Here we show by ion current measurements that there are two distinct light-regulated inward currents which are localized in the eyespot and in the flagellar region of the cell. From the kinetics and the rhodopsin action spectrum of these photocurrents we conclude that they are part of the rhodopsin-regulated signal transduction chain controlling the cellular behaviour in light. Both photocurrents are Ca²⁺-dependent and are suppressed by the Ca²⁺-channel inhibitors verapamil and pimozide, suggesting that the photoreceptor current and probably the flagellar current are both carried by Ca²⁺.