In comparison to prokaryotic picoplankton, our knowledge of the diversity of eukaryotic picoplankton is still very limited. Recently, clone libraries from the small sub-unit ribosomal RNA gene (rDNA) from oceanic samples have revealed the presence of many new eukaryotic groups, not represented in cultures (Lopez-Garcia et al. 2001. Unexpected diversity of small eukaryotes in deep-sea Antarctic plankton. Nature 409:603-607; Moon-van der Staay et al.. 2001. Oceanic 18S rDNA sequences from picoplankton reveal new eukaryotic lineages. Nature 409: 607-610). In the frameworkof the European program PICODIV, we are using the same approach to characterize picoplankton in coastal waters. In addition to clone libraries, we are also attempting to isolate novel strains in culture and to quantify key groups with oligonucleotides probes detected by fluorescence in situ hybridization (FISH). Some of the novel groups already observed in the Pacific Ocean are found in coastal waters pointing out their ubiquity. One such group, probably heterotrophic, occupies a basal position within the stramenopiles. Two other major novel groups are part of the alveolates and appear to be intermediate between dinoflagellates and apicomplexa. Among the photosynthetic groups, Mamiellales (Prasinophyceae), cryptophytes, chryso phytes and dinoflagellates dominate coastal clone li braries. Some sequences can be relegated to the novel class Bolidophyceae, previously known only from off shore waters. Among heterotrophic groups, ciliates are particularly abundant. Finally, coastal waters seem to harbor some novel eukaryotic groups, not observed previously such as one that appears close to the Rhodophyta.