Toxin producing dinoflagellates of the genus Alexandrium Halim represent a risk to Arctic environments and economies. This study provides the first record and a characterization of Alexandrium ostenfeldii in the western Arctic. During a cruise along the coasts of western and southern Greenland 36 isolates of the species were established in August 2012. Plankton samples taken at 3 different stations from the upper water layer at water temperatures of approx. 4-7 °C, contained low amounts of A. ostenfeldii. Sequencing of SSU and ITS-LSU rDNA and subsequent phylogenetic analyses identified all Greenland strains as members of a NW Atlantic spirolide producing phylogenetic clade. Molecular results were confirmed by morphological features typical for this group (= Group 5 of a recent ITS-LSU phylogeny of A. ostenfeldii). The Greenland isolates did not contain either Paralytic Shellfish Poisoning toxins or gymnodimines, but produced several spirolides. Altogether 12 different analogues were detected, of which only SPX-1, C, 20-meG and H have been described earlier. The remaining 8 spirolides have not been identified so far. Some of them were found to dominate the toxin profiles of a number of isolates. Among the 36 investigated strains spirolide composition varied considerably, particularly isolates from western Greenland (Station 516) exhibited a high diversity of analogues, with different profiles in nearly all 22 isolates. All of the 34 tested Greenland strains showed considerable lytic capacity when exposed to Rhodomonas salina.