Aquatic organisms present, not only simple sources of accumulated metal, but can interact with metals, altering their toxicity. Due to exposition of biosphere with metals, organisms have developed various defense mechanisms to protect themselves against adverse effects of these ions and their compounds. Metallothionein (MT) is one of that which represents a critical mechanism for detoxification of metals.The sterlet (Acipenser ruthenus) is a bottom feeding sturgeon specie and because the fish are dependent on invertebrate species for food throughout their life cycle, the sterlet could be a good indicator of the quality of the state of water ecosystem. Addition of copper to water leads to the induction of MT. The present study analyzed MTgene that was excreted from the liver of sterlet exposed to sub-lethal copper concentrations (0.075 mgL-1). To begin to elucidate the molecular mechanism(s) of sensitivity of sturgeons to metals, a RNA encoding MT was purified from livers of sterlet, then a cDNA was synthesized and the MTgene was amplified. The primary structure of sterlet metallothionein (S-MT) contained 20 cysteine residues, which is the same as MTs of teleost fishes. However, the primary structure of S-MT contained 63 amino acids, which is longer than any MT identified in teleost fishes but similar to Lake sturgeon and White sturgeon. The complete nucleotide sequence of the S-MTgene has been detected. We have determined the structure of the fish copper-binding protein by DNA sequence analysis of the gene.