The genus Diadesmis (Bacillariophyta) in the Crozet and Kerguelen Archipelagos was studied using light and scanning electron microscopy. Samples were collected from different habitats: lakes, temporary peat-pools, mosses and soils. Ten species have been recorded. The typification of the species Diadesmis arcuata Heiden, described from Kerguelen Archipelago, is reviewed. Five new species are described: Diadesmis comperei, Diadesmis costei, Diadesmis crozetikerguelensis, Diadesmis langebertalotii and Diadesmis subantarctica. The subgenus Paradiadesmis appears to have an unusual structure: the interstriae are quadrangular hollow tubes.