A taxonomic account of the sea ice diatom,Gyrosigma concilians, is presented with the designation of a neotype from modern collections. Detailed morphological data of Canadian Arctic specimens, based on light (LM) and, for the first time, scanning electron (SEM) microscopy, confirm the slightly oblique nature of the transapical striae which are intersected by a more finely striated longitudinal pattern, and the presence of small, thin central bars on the internal valve face. Valves of G.conciliansrange from 57–109 μm in length, 6.5–11 μm in width and have 22–27 transapical striae in 10 μm crossed by a closer longitudinal pattern of 38–44 in 10 μm. Relationships with another species in the genus are discussed and the circumpolar Arctic distribution of G.conciliansis d