A new marine diatom, Nitzschia navis-varingica, sp. nov., isolated from Vietnamese waters, is described by light, transmission, and scanning electron microscopy, including thin sectioning. The new species has been found to produce the neurotoxin domoic acid (DA), better known from several species of Pseudo-nitzschia Peragallo and one species of Amphora Ehrenberg. Production of DA is therefore more widespread among diatoms than previously thought. Taxonomically, the genus Nitzschia Hassall is exceptionally difficult, with about 900 described taxa. Grunow (in Cleve and Grunow 1880 divided the genus into 24 sections, and this system is still used with modifications. Nitzschia navis-varingica, sp. nov. fits best into a group of sections that includes Dubiae, Bilobatae, most of the Lanceolatae, and Lineares, all sensu Grunow, as the cell is slightly indented in the middle in girdle view and has a moderately eccentric raphe and a weak longitudinal fold on the valve. Many species within these sections have features similar to N. navis-varingica, but no species seems to be identical. Because both Pseudo-nitzschia and Nitzschia belong to the family Bacillariaceae, it seems reasonable to look for further producers of DA in this family, including freshwater species, which mainly comprise species within the sections Dubiae, Bilobatae, Lanceolatae, and Lineares.