ABSTRACT.    Hydrothermal vents represent one of the most extreme ecosystem, in which environmental conditions vary tremendously both in space and time, leading to a very high level of species endemism. In contrast to hypotheses about life antiquity initially proposed at the discovery of these communities, we now know that benthic species living there mostly represent a derived modern fauna which has evolved following secondary adaptations from shallower organisms. Does this assumption also hold for smaller eukaryotes, such as microscopic ones? By direct observations under microscope, we detected a very dense community of protists growing inside the mantle cavity waters of several species of hydrothermal bivalves belonging to the genera Bathymodiolus and Calyptogena. Mechanisms by which protists accumulate are still uncertain (passive concentration caused by the filtering activity of these bivalves, symbioses, parasitisms, or "organismal reactor" where protists may reach their optimal growth conditions?). In order to assess the level of protist community's complexity, we decided to analyze the genetic diversity of samples collected from vent bivalves, as a first attempt to get an exhaustive inventory of the microeukaryots. For that purpose, we performed a systematic sequencing of the small ribosomal subunit (18S), from samples collected along both the East Pacific Rise (EPR) and the Mid-Atlantic Ridge (MAR). Our first results showed a very high level of lineage diversity, which represent a subset of the main eukaryotic lineages known to date elsewhere. In particular, genetic libraries obtained from the EPR are dominated by sequences of Ciliophora, Cercozoa, and Fungi.