This is the first record of a domoic acid-producing organism from polar waters. Strains of the cold-water diatom Pseudo-nitzschia seriata were isolated from water samples collected from the west coast of Greenland. Growth experiments on two of the strains showed similar growth rates at 4°C and production of DA (domoic acid) and the two isomers IA (isodomoic acid A) and IB (isodomoic acid B). Both strains produced significantly higher amounts of DA than of IA and IB. The concentrations of domoic acids were 1.46–1.93pgpercell, and thus have the potential to pose a risk for amnesic shellfish poisoning.Four strains of P. seriata were cultured at different temperatures (4, 10 and 15°C), and morphological differences were found at the different temperatures. Similar differences have been observed previously in field samples from different geographical areas and have resulted in uncertainty regarding the taxonomy of P. seriata and related species. Transmission electron microscopy showed that at 4°C the strains corresponded to the original description of P. seriata. At higher temperatures, the number of rows of poroids in the striae as well as the density of poroids decreased, and cells then resembled the closely related P. australis. The Greenlandic strains of P. seriata did not grow well at 15°C. The identity of the strains was confirmed by analyses of ITS1, 5.8S and ITS2 rDNA from P. seriata and P. australis, using strains from different geographical areas. There was a close phylogenetic relationship between P. seriata and P. australis but the analyses demonstrated consistent molecular differences between the two species, in spite of the morphological similarity.