Metallothioneins (MTs) are small essential proteins involved in cellular processes of metal handling and detoxification. Here, we describe the structure of 2 MT genes in a marine mollusc, the European flat oyster Ostrea edulis. The sequences of these 2 genes code for 2 proteins of 74 and 71 amino acids, respectively, with the 71 amino acid protein having an abnormal lack of cysteine residues. The direct quantification of metal ions bound to purified recombinant proteins demonstrated that the metal-binding capacity of the 2 proteins differs by about 20%. An enzyme-linked immunosorbent assay (ELISA) test, used to quantify MT protein expression in O. edulis exposed to metal under laboratory conditions, showed no significant induction of MTs, either in the gills or the digestive gland despite an increase in metal concentration observed in the same tissues. Similar results were observed with an MT-RNA expression study. These results seem to indicate a low involvement of MTs in metal detoxification in O. edulis. Examination of polymorphism in the coding sequence of OeMT (O. edulis metallothionein) genes by single-strand conformation polymorphism (SSCP) revealed a lack of genetic diversity in field populations.