Thalassiosira bulbosa Syvertsen is described from one clonal culture and natural marine, arctic material collected from brown spots on the ice and from seawater close to the ice border. The new taxon belongs to the group of Thalassiosira species with a single central strutted process and one marginal ring of strutted processes. Specific features are the conspicuous bulb-shaped outer part of the marginal strutted processes and a strutted tube surrounded by two oblong, bean-shaped satellite pores. T. bulbosa forms "resting cells" differing from the vegetative cells mainly in the degree of silicification and by having fewer openings through the frustule. The species may obtain its maximal cell size by a purely vegetative auxospore formation.