Previously, Navicula ostrearia B. has been described only from the Marennes-Oleron district of the Atlantic coast of France. However, this pennate diatom is also present In the Bay of Bourgneuf where breeding of green-gilled oysters were recognized in sea basins called oyster ponds. A short description of observations made with the optical microscope precedes description of high magnification examination of the frustule made with the scanning electron microscope. This combination of techniques shows a polymorphism between the external and internal surfaces of diatom valves.