ABSTRACT: The vertical and regional distribution of Triparma laevis (Parmales), a siliceous pico-sized eukaryotic marine phytoplankton species, was investigated during summer off the south coast of Hokkaido, Japan, in the western North Pacific. Growth characteristics were also studied in the laboratory using a recently isolated culture strain. T. laevis was abundant in the subsurface layer (30 to 50 m), where water temperature was <10°C, but it was absent above the pycnocline when temperatures were >15°C. Growth experiments revealed that T. laevis was able to grow at 0 to 10°C but not higher than 15°C, indicating that its depth distribution mainly depended on temperature. High irradiances resulted in increased growth rates of T. laevis, with the highest rates of 0.50 d⁻¹ at 150 µmol m⁻² s⁻¹. Using measured daily incident photosynthetically available radiation and in situ light attenuation, the growth rates of T. laevis at 30 and 50 m were calculated as 0.02 to 0.34 and −0.01 to 0.08 d⁻¹, respectively. Distribution of T. laevis was strongly coupled with the occurrence of Oyashio water. The growth rate of T. laevis was lower than that of bolidophytes and diatoms, which are closely related to Parmales. Bloom forming diatoms in this region have higher growth rates and a broader temperature range (0 to 20°C) and, therefore, might be expected to dominate, but Parmales does manage to co-exist in this region, albeit at a moderate abundance compared to the diatoms.