Metallothioneins (MT) play important roles in trace metal homeostasis as well as excess metal detoxification in all organisms. They have been acknowledged as the most valid biomarker to indicate metal exposure in both vertebrates and invertebrates. In this study, the coding sequence of MT of a mollusk, the oyster Crassostrea hongkongensis, was determined by molecular cloning. The derived peptide sequence is composed of 75 amino acid residues of high cysteine content (28%). Blast analysis indicated that the MT gene in C. hongkongensis shares high homology with the MT genes of other aquatic invertebrates. The prokaryotically expressed ChMT protein was purified and used as antigen in the development of polyclonal antibodies, allowing the quantification of ChMT expression by competitive enzyme-linked immunosorbent assay in various tissues of the oyster on CdCl₂-stressed conditions. Clear inductions of MT expression were observed in the digestive gland, gills as well as mantle by CdCl₂ at a concentration of either 100 or 300 µg/l, notably from 24 h to 8 days posttreatment. Indicating a dosedependent manner, CdCl₂ in 300 µg/l scale always led to more pronounced MT enhancement. The MT induction profile covered three stages of accumulation, gradual decline, and sharp decline, which likely indicates the differential physiological states of C. hongkongensis on continuous cadmium exposure. Our study demonstrates for the first time that the MT in C. hongkongensis could potentially respond to the presence of Cd in aquatic environments, and developed a practical approach that can efficiently and specifically detect ChMT from the oyster tissues. The ChMT induction profile on high level of Cd highlights the relationships between MT inducibility and the cellular compensatory and/or toxic responses.