Terrestrial helicid snails (Helicidae) accumulate and store huge amounts of metallic trace elements (Cu, Cd) directly from their environment by binding the metals to Metallothioneins (MTs). Previous studies with different helicid species (Helix pomatia, Arianta arbustorum) have revealed that these animals possess two strictly metal-specific MT isoforms. While one of them is directly involved in the maintenance of intracellular Cu+ homeostasis, the second isoform is strictly associated with Cd2+ and plays an important role in detoxification of this harmful metal. The purpose of the present study was to identify and characterizeMT isoforms from another helicid species, Cornu aspersum. Three full-length MTcDNAswere characterized from this species.Two of them exhibited a high degree of identity with the formerly known Cu+- and Cd2+-specific MT isoforms from Helix pomatia. The third isoform isolated from C. aspersum seems to be associated with both metals (Cd and Cu). Analysis of the translated amino acid sequence of the intermediate Cd/Cu isoform exhibited major differences in the primary sequence compared to the other two isoforms. The three amino acids Arg, Lys and Asn (prevailing in the other two isoforms) were replaced by the neutral amino acid Ser. We suggest therefore, that the molar content and position of the amino acids mentioned within the sequence of the three isoforms may contribute significantly to metal specificity. Computational analysis of several biochemical parameters indicates, moreover, that the novel Cd/Cu isoform may represent an intermediate evolutionary stage between the two other, metal-specific isoforms.