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9 pages, 4 figures, 4 tables, The class Prasinophyceae (Chlorophyta) contains several photosynthetic picoeukaryotic species described from cultured isolates. The ecology of these organisms and their contributions to the picoeukaryotic community in aquatic ecosystems have received little consideration. We have designed and tested eight new 18S ribosomal DNA oligonucleotide probes specific for different Prasinophyceae clades, genera, and species. Using fluorescent in situ hybridization associated with tyramide signal amplification, these probes, along with more general probes, have been applied to samples from a marine coastal site off Roscoff (France) collected every 2 weeks between July 2000 and September 2001. The abundance of eukaryotic picoplankton remained high (>103 cells ml-1) during the sampling period, with maxima in summer (up to 2 × 104 cells ml-1), and a single green algal species, Micromonas pusilla (Prasinophyceae), dominated the community all year round. Members of the order Prasinococcales and the species Bathycoccus prasinos (Mamiellales) displayed sporadic occurrences, while the abundances of all other Prasinophyceae groups targeted remained negligible, This work was funded by the following programs: PICODIV, supported by the European Union (EVK3-CT-1999-00021); PICMANCH, supported by the Région Bretagne; and BIOSOPE, supported by PROOF (CNRS). F.N. was supported by a doctoral fellowship from the French Research Ministry