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, while 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 μmol m−2 s−1) or decreases (700 nm, 18.4–36.0 μmol 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 μmol m−2) had a higher absorption in the red region of the absorption spectra than those cultured in white light (92.7 μmol m−2).