Metallothioneins (MTs) are considered central in the intracellular regulation of metals such as copper, zinc, and cadmium. Increased MT synthesis is associated with increased capacity for binding these metals and protection against metal toxicity. Recent advances in the biochemistry and molecular biology of MTs have facilitated research on MTs in aquatic species. For the bivalve mollusc Crassostrea virginica, a species frequently used in studies on the toxicology and environmental monitoring of metals, the primary structure for MT has been deduced from analysis of the proteins and cDNA. Procedures for analysis of MT synthesis and MT gene expression have been applied in studies of response to metal exposure. Induction of specific MT forms by Cd is concentration-and time-dependent. The levels of MT-bound metals exhibit a strong relationship with the cytosolic metal concentrations in a metal-exposed natural population of oysters. Ribonuclease protection assays using sequence-specific antisense RNA probes have shown that the MT mRNA structure in this natural population exhibits considerable individual variability in the 3'-untranslated region. Although yet to be substantiated, the possibility exists that the distribution of this variability may be related to the level of environmental metal contamination. (3,20,21) attests to the conserved nature of MT and its function. Amino acid sequences of varied species including aquatic animals show regions of high similarity (22). Conserved nucleotide sequences exist for both coding region and regulatory elements of MTs of mammals, fish, and invertebrates (23-25).The initial reports describing the occurrence of MTs in aquatic animals (26,27) were soon followed by proposals for use of MTs in assessing the environmental toxicity of metals (28,29). These early proposals were considered controversial due to a lack of general understanding of MT function in aquatic animals. While it is has been known for some time that metals such as Cd, Zn, and Cu can induce MTs in aquatic animals, detailed studies at the cellular and molecular level have only recently been reported (21 (22,25,(33)(34)(35)(36)(37)(38)(39)(40)(41)(42)(43)