Ostreococcus tauri Courties et Chrétiennot-Dinet is the smallest described autotrophic eukaryote dominating the phytoplanktonic assemblage of the marine Mediterranean Thau lagoon (France). Its taxonomic position was partly elucidated from ultrastructure and high-pressure liquid chromatography (HLPC) pigment analysis. The sequence analysis of the 18S rDNA gene of O. tauri measured here is available in EMBL Nucleotide Sequence Database (accession number: Y15814) and allowed to clarify its phylogenetic position. O. tauri belongs to the Prasinophyceae and appears very close to Mantoniella, a typical scaly Prasinophyceae, morphologically very different from the naked and coccoid Ostreococcus. An electrophoretic analysis of O. tauri shows that the nucleus contains 10.20 mbp. This small genome, fragmented into 14 chromosomes ranging in size from 300 to 1500 kbp, confirms the minimalist characteristics of Ostreococcus tauri.