Physiological studies in recenet years have shown the high adaptability from freshwater to seawater of of Javanese Medaka, Oryzias javanicus. Results indicate O. javanicus is a unique and good candidate as a test fish in both marine and freshwater environments and may deliver new insights into our knowledge of aquatic risk assessment. To develop molecular biomarkers for toxicity assessment, we have obtained six stress-responsive genes [metallothionein (MT), catalase (CAT), superoxide dismutase (SOD), glutathion S-transferase (GST), cytochrome P-450 (CYP1A1), uniquitin (UBI)] from O. javanicus and investigated their expression changes in fishes exposed to a variety of concentrations of cadmium (Cd). More detailed observations on differential expression of MT gene after heavy metal exposure were carried out as follows. The pattern of OjaMT expression induced by six heavy metals was analyzed by real-time quantitative PCR. The level of hepatic OjaMT mRNA was increased in a dose-dependent manner by Ag, Cd, Cu, and Zn after 24 h of exposure. However, after Cr and Ni exposure, a significant decrease in OjaMT levels was observed. Cadmium-induced OjaMT expression was detectable in fishes as young as 3 months. After Cd exposure, OjaMT induction was prominent in intestine and liver and intermediate in muscle and gill. OjaMT mRNA levels could representa good biomarker for monitoring heavy metals in seawater.