Recently, we discovered the LOV-containing blue light (BL) receptor, aureochrome, in the stramenopile alga Vaucheria. Aureochrome has been isolated in other stramenopile algae, such as Fucus and Ochromonas. The functional role of aureochrome has still not been identified in Fucus and Ochromonas. What kind of response does aureochrome mediate? Here, we investigated various light responses, such as cell morphology, motility and behavior in Ochromonas danica, by measuring cell dimensions and observing swimming behavior under various light conditions. Although cell length was not changed by different wavelengths of light (BL, GL and RL), cell width decreased in BL and GL in comparison with RL; that is RL-cultured cells are more spherical relative to those cultured in BL or GL. Also, the swimming speed of O. danica is slower under RL than under BL or GL. There was also an accumulation response toward a small irradiated area, but this response was induced only by BL or GL, not RL. The possible involvement of photoreceptors absorbing BL and GL in these morphological and motility responses is discussed.