Round, brown, spiny dinoflagellate cysts characterise many modern high-latitude assemblages. Abundance is often highest where summer sea-surface temperatures seldom exceed 7 °C and where winter sea-surface temperatures are around 0 °C, making this morphological group important for reconstructing cold intervals within marine Quaternary sequences. Our analysis of modern sediments from the Beaufort Sea of Arctic Canada, the Kara and Laptev seas of Arctic Russia, and across the Arctic Ocean, allows us to recognise the new cyst genus Islandinium along with the extant species Islandinium minutum (Harland and Reid in Harland et al., 1980) new combination (basionym: Multispinula? minuta), Islandinium? cezare (de Vernal et al., 1989 ex de Vernal in Rochon et al., 1999) new status and combination (basionym: Multispinula? minuta var. cezare) sensu lato, and Echinidinium karaense new species. Of these, the generotype Islandinium minutum is a well-known but morphologically problematic species. We have re-examined the type material from the Beaufort Sea and studied specimens from across the Arctic, and our observations clarify ambiguities in the original description of this species. The archeopyle of Islandinium minutum results from the loss of three apical plates, an unusual style among peridiniphycidean dinoflagellates. The asymmetrical location of these plates around the apex is distinctive, and probably contributed to earlier misunderstandings of the archeopyle. Previous attributions to Multispinula? and Algidasphaeridium? are unsustainable. Maps showing the distribution of Islandinium minutum are given for the Northern Hemisphere and show this species to be polar to north-temperate, whereas Islandinium? cezare s.l. and Echinidinium karaense appear to be more restricted to polar environments. Copyright © 2001 John Wiley & Sons, Ltd.