To gain basic insights into the marine pelagic diatom species Neodenticula seminae, and particularly the relationships between morphology, evolution and environment, we conducted a scanning electron microscopic image analysis using modern specimens collected in the subarctic North Pacific and Bering Sea during the summers of 1955 and 1996–2003. We recognized an apparent intraspecific variability between two biogeographic provinces in terms of morphology: (1) the Oyashio Region (in the northwestern margin of the Pacific Ocean) and (2) other oceanic regions. This variability may reflect spatial variation in the availability of nutrients such as silicate. Specimens from the Oyashio morphological province had thinly silicified valves, whereas those from the other oceanic provinces were extraordinarily rich in heavily silicified valves.