To elucidate the phylogeny of the Dunaliellales sensu Ettl. the taxon often thought to be intermediate between primitive green flagellates such as the prasinophytes and the advanced chlorophycean algae, the sequences of the nuclear-encoded small subunit ribosomal RNA gene (18SrDNA) were determined and analyzed for five green algae, including three dunaliellalean algae. Phylogenetic trees based on 18SrDNA suggest that Oltmannsiellopsis viridis (Margraves et Steele) Chihara et Inouye represents an early divergence in the Ulvophyceae/Trebouxio-phyceae/Chlorophyceae clade and has no close relationship to any other green algae, as also suggested from ultrastructural characters. We propose Oltmannsiellopsidates ord. nov. for this genus. Hafniomonas and Polytomella are included in the clade which is characterized by clockwise basal bodies (CW group). The 18SrDNA trees suggest that multiple losses of the cell wall of the flagellate cell occurred in the CW group, and that the Dunaliellales sensu Ettl has a polyphyletic nature. This study also suggests that Planophita terrestns Groover et Hof-stetter (Chaetopeltidales) and Chaetophora incras-sata (Hudson) Hazen (Chaetophorales) are distinct lineages in the Chlorophyceae.