At fluence rates below 45 W· m-2 cells of the flagellate stage of Haematococcus lacustris react only positively phototactically with a rather high degree of orientation (indicated by r values up to 0.66 with the Rayleigh test). The directedness of orientation decreases with decreasing irradiance. The degree of directedness of the phototactic response depends on the intensity of preirradiation: Low light intensity applied after strong light application results in a “dark reaction” (low r values), low light given after darkness stimulates a rather high degree of directedness of positive phototaxis. Weak blue light (λ=483 nm; 0.4 W · m-2) stimulates positive phototactic response, whereas comparable red light (λ=658 nm; 0.5 W · m-2) does not.