The phototactic orientation in Gyrodinium dorsum and G. aureolum has been analyzed. Both species show only positive phototaxis with an optimum at about 300 W-m−2. The mechanism of photoorientation does not seem to be based on a dichroic orientation of the photoreceptor pigments as in the flagellate Euglena gracilis. Photobleaching experiments have shown a far higher resistance toward continuous irradiation at even high fluence rates than in other flagellates which is in good agreement with the exclusive behavior of positive phototaxis.