Abstract
Stephanodiscus species from central and northern Canadian lakes have been investigated and their morphology illustrated with the help of light and electron microscopy. Stephanodiscus vestibulis Håkansson, Theriot & Stoermer and Stephanodiscus hantzschii fo. tenuis (Hustedt) Håkansson & Stoermer are identified with certainty. Morphological variation in the S. niagarae-complex and the S. alpinus-complex is discussed and a new species, S. agassizensis sp. nov., is described. S. subtranssylvanicus Gasse and S. oregonica (Ehrenb.) Håkansson, previously considered extinct, are reported from living material.