The marine diatom Haslea ostrearia Simonsen produces a blue pigment, marennine, which is used for greening oysters. This microalga is cultured industrially indoors with artificial light. The influence of light quality on marennine production by cultures of H. ostrearia was investigated in the laboratory and at a semi-pilot scale (300 L tanks). In the first series of experiments in the laboratory, a clone of H. ostrearia was cultured under light of different colors (white, blue, green, yellow, and red) and at two irradiances ('low' and 'high', 20 and 100 µmol photons m −2 s −1 , respectively). Compared to the white light controls, growth was increased in blue light at the 'low', but not at the 'high' irradiance, and marennine production at the end of the exponential phase was the highest in cells grown under blue light, regardless of the light quality or intensity during growth. Increased marennine production during growth was also observed, whichever color of light (blue or white) was used during the acclimation phase. In a second series of experiments, intraclonal differences were studied by comparing marennine production in seven clones differing with regard to their mean cell size. The total marennine expressed either per cell or per culture volume, was higher in blue light for all the clones. Complementary experiments carried out under semi-industrial conditions confirmed this effect of blue light, which could be relevant for the industrial, indoor production of marennine.Abbreviations: BL, blue light; WL, white light