During a screening of pennate diatoms for species capable of producing domoic acid, a previously undescribed species of Plagiotropis, P. tristis sp. nov., was found. The culture was isolated from a plankton sample off Tromsø, Norway. Plagiotropis tristis is distinguished from other species of Plagiotropis by the lack of a valve fold, the low raphe ridge, the type and distribution of areolae and the internal and external polar raphe ends. The raphe structure resembles two hands grasping each other by the fingers in transverse thin sections. With regard to ultrastructure, many of the features of P. tristis are common to pennate diatoms, and the pyrenoid is transversed by two thylakoid-like membranes. Plagiotropis tristis is found to produce mucilage, and several vesicles filled with fibrillar material that might be involved in mucilage production were observed. This is the first report on the ultrastructure of a Plagiotropis species. A culture of P. tristis was screened for domoic acid, and was not found to produce the toxin in a detectable amount.