A microalga was isolated from the pack-ice of Wood Bay (Antarctica) during austral summer 1993–4. By comparison with an authentic strain obtained from Sammlung von Algenkulturen at the University of Göttingen (SAG 860–3), its ultrastructural details and pigment composition were very similar to those of Pleurochloris meiringensis. However, sequence analysis of the entire 18S rRNA gene revealed substantial genetic divergence (57 nucleotide substitutions and 19 insertions/deletions) between SAG 860–3 and the Antarctic isolate. Phylogenetic analysis of complete 18S rRNA gene sequences from 18 microalgal species, including the two strains studied, rejected the hypothesis that the Antarctic isolate is the sister species to Pleurochloris meiringensis. Molecular evidence indicates that the Antarctic isolate is the sister group of a clade that comprises two other representatives of the Xanthophyceae (synonym Tribophyceae): Pleurochloris meiringensis and Botrydiopsis intercedens. This leads us to conclude that this microalga is a new species belonging to a new genus and the name Pseudopleurochloris antarctica is proposed.