We used sea urchin embryos as bioindicators to study the effects of exposure to sublethal cadmium concentrations on the expression of the metallothionein (MT) gene stress marker. For this purpose, the complete complementary deoxyribonucleic acid of the species Paracentrotus lividus (Pl) was cloned and sequenced. Northern blot analysis showed that basal levels of Pl-MT messenger ribonucleic acid, having an apparent size of 700 bases, are expressed in all developmental stages analyzed, from early cleavage to pluteus. However, when embryos were continuously cultured in sublethal CdCl₂ concentrations and harvested at cleavage, swimming blastula, late gastrula, and pluteus stages (6, 12, 24, and 48 hours after fertilization, respectively), a time- and dose-dependent increase in the transcription levels of the Pl-MT gene was observed. Interestingly, although microscopical inspection revealed the occurrence of abnormalities only after 24 hours of exposure to the pollutant, Northern blot and reverse transcriptase–polymerase chain reaction analyses revealed significant increases in Pl-MT expression levels already after 12 and 6 hours of exposure, respectively. Therefore, this study confirms the validity of MT as marker of exposure and provides evidence that Pl-MT and sea urchin embryos can be a potentially valuable and sensitive model for testing in very short periods of time seawaters heavily contaminated with cadmium.