Abstract
A new species, Stephanodiscus yukonensis sp. nov. (Thalassiosiraceae), is described from an arctic lake in the Yukon Territory, Canada. Unusual for the genus Stephanodiscus, this new species has two rimoportulae in different positions on the valve with external openings which also differ in form. Its relationship within the genus Stephanodiscus and to other genera in the family Thalassiosiraceae is discussed.