A new Orthoseira species, Orthoseira gremmenii sp. nov., is described from the small volcanic Gough Island, located in the southern Atlantic Ocean. Using light and scanning electron microscopy, the discriminating features of the species are revealed. Orthoseira gremmenii is characterized by its very large valve dimensions (total valve diameter up to 100 μm), the presence of a large hyaline central area (more than 50% of the total valve diameter), the occurrence of regularly placed bifurcated linking spines and the absence of typical features such as undulations in the valve interior (so-called caverns) and marginal porefields, commonly reported in other species. A very small proportion of the population possesses typical internal valves, covering the entire valve interior, leaving only a few circular, window-like openings close to the valve face/mantle junction. The presence of these internal valves remains so far without explanation but may be related to the extreme environmental (i.e. aerophilic) circumstances in which the species is found. © 2008 Adac.