Abstract Benthic dinoflagellates of the genus Ostreopsis are found all over the world in temperate, subtropical, and tropical coastal regions. Our recent studies revealed that a putative “cryptic” species of Ostreopsis ovata is present widely along Japanese coasts. This organism, Ostreopsis sp. 1, possesses palytoxin analogs and thus its toxic blooms may be responsible for potential toxification of marine organisms. To evaluate the bloom dynamics of Ostreopsis sp. 1, the present study examined the growth responses of Ostreopsis sp. 1 strain s0716 to various light intensities (photon flux densities: μmolphotonsm−2s−1) using a newly devised photoirradiation-culture system. This novel system has white light-emitting diodes (LEDs) capable of more closely simulating the wavelength spectrum of light entering the oceanic water column than do fluorescent tubes and halogen lamps. In this system, the light intensity of the white LEDs was reduced through two polarizing filters by varying the rotation angles of the filters. Thereby, the new system was capable of culturing microalgae under well-controlled light intensity conditions. Ostreopsis sp. 1 grew proportionally when light intensity was increased from 49.5 to 199μmolphotonsm−2s−1, but its growth appeared to be inhibited slightly at ≥263μmolphotonsm−2s−1. The relationship between observed growth rates and light intensity was calculated at R>0.99 (P<0.01) using a regression analysis with a modified equation of the photosynthesis-light intensity (P-L) model. The equation determined the critical light intensities for growth of Ostreopsis sp. 1 and the organism's growth potential as follows: (1) the threshold light intensity for growth: 29.8μmolphotonsm−2s−1; (2) the optimum light intensity (Lm) giving the maximum growth rate (μmax=0.659 divisions day−1): 196μmolphotonsm−2s−1; (3) the optimum light intensity range (Lopt) giving ≥95% μmax: 130–330μmolphotonsm−2s−1; (4) the semi-optimum range (Lsopt) giving ≥80% μmax: 90 to over 460μmolphotonsm−2s−1. The Lsopt represents 4.5–23% ambient light intensity present in surface waters off of a temperate region of the Japanese coast, Tosa Bay; putatively, this semi-optimum range of light intensity appears at depth of 12.9–27.8m. Considering these issues, our data indicate that Ostreopsis sp. 1 in coastal environments may form blooms at ca. ∼28m depth in regions along Japanese coasts.