The unusual characteristics of the hydrothermal vent environment (high pressure and metal concentrations, low pH, etc.) leads us to wonder how species living in this particular biotope have adjusted to these severe living conditions. To investigate the consequences of high metal concentrations, filter-feeding organisms are commonly used in ecotoxicological studies. Metallothioneins (MTs) are proteins conserved throughout the animal kingdom and involved in intracellular metal regulation. Therefore, we tried here to find out whether the metallothioneins of hydrothermal bivalves are different from those of coastal bivalves. The characterization of DNA sequences coding MTs from some of the most common hydrothermal bivalves, belonging to the genus Bathymodiolus (Mytilidae) was performed by reverse transcription polymerase chain reactions (RT-PCRs). The complementary DNAs (cDNAs) of MT-10 and MT-20 isoforms were obtained for the Atlantic and Pacific hydrothermal mussels (Bathymodiolus azoricus and Bathymodiolus thermophilus). The MT-10 transcripts were 222 nucleotides long and the MT-20 transcripts, 207 nucleotides. The polymorphism of the MT cDNAs in these two hydrothermal species is discussed. The comparison between metallothionein cDNA sequences of the Mytilus and the Bathymodiolus genera shows strong homologies among metallothioneins of coastal and hydrothermal mussels.