Gyrodinium dorsum Kofoid responds photophobically to flashes of blue light. The photophobic response consists of a cessation of movement (stop-response). Without background light and after a flash fluence above 10 J m-2, 75-85% of the cells show a stop-response, only only 50% of the cells show this response at 5 J m-2. With a flash fluence of 5 J m-2, background light of different wavelengths either increases (614 nm, 5.5-18.2 .mu.mol m-2 s-1) or decreases (700 nm, 18.4-36.0 .mu.mol m-2 s-1) the stop-response. Two hypotheses for the mechanism of the modulation by background light of the photophobic response are discussed: an effect of light on the balance of the photosynthetic system (PS I/PS II) or an effect on a phytochrome-like pigment (Pr /Pfr). This study supports the idea that a phytochrome-like pigment works in combination with a blue light-absorbing pigment. It was also found that cells of Gyrodinium dorsum cultured in red light (39.8 .mu.mol m-2) had a higher absorption in the red region of the absorption spectra than those cultured in white light (92.7 .mu.mol m-2).