Sexual reproduction is an obligatory phase in the life cycle of most diatoms, as cell size decreases with successive vegetative divisions and the maximal cell size is only restored by a specialized cell, the auxospore, which follows zygote formation as a result of sexual reproduction. While in pennate diatoms the induction of sexual reproduction depends primarily on cell-cell interactions, the importance of different external factors for the induction of sexual reproduction is less well known. Here, we investigated the effects of light on sexualization in the marine benthic pennate diatom Haslea ostrearia (Gaillon) R. Simonsen. Compatible clones were crossed and exposed to different combinations of light levels, qualities, and photoperiods. Light was found to be a key factor for sexualization, and to a certain extent, to control auxosporulation in H. ostrearia. The light conditions most favorable for sexual reproduction were low irradiances (<50 micromolphotons m(-2) s(-1)) and short photoperiods (6-10 h), conditions that prevail during winter, and to a lesser extent, the higher irradiances and longer photoperiods that correspond to the spring and fall, when blooms of this organism form in the natural environment. Auxospore formation was very rare in continuous light, and maximum in presence of red radiation, while it was never observed in darkness or in radiation other than red.