Metallothioneins, metal-inducible proteins, are being characterized from different organisms and shown as potential biomarkers of exposure to pollution by certain heavy metals. Here we report the identification of a new metallothionein cDNA (433bp) from the shrimp Macrobrachium rosenbergii, putatively encoding a 61 residue polypeptide. Tissue specific analysis indicated that Mar-MT-I (M. rosenbergii Metallothionein Gene-1) is expressed with the highest levels in the hepatopancreas and lowest in the thoracic ganglia, and none in the gills or muscles. In addition, our data showed that Mar-MT-I is differentially regulated in the hepatopancreas by certain heavy metals and thermal stress: Cd and Cu produce somewhat similar expression profile patterns, Zn has a reductional effect and thermal stress alone entirely stops its expression. These results show that Mar-MT-I mRNA levels can potentially be used as biomarkers for Cd, Cu or Zn pollution individually. However, in the case of combined metal treatment, different combinations of these metals have quite different effect on Mar-MT-I expression. Therefore, factors of such differential behaviors should be kept as a priority for further biomonitoring studies.