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<i>Phascolosoma esculenta</i>, an economically important species inhabiting the high tide areas of the intertidal zone, is particularly sensitive to water pollution. Considering its potential as a bioindicator, studies on the ecotoxicology of <i>P. esculenta</i> are imperative. The toxic effects of cadmium (Cd) were analyzed by exposing <i>P. esculenta</i> to different concentrations of Cd (6, 24, 96 mg/L). In this study, the changes in the antioxidative indexes of total superoxide dismutase (T-SOD), glutathione s-transferase (GST), reduced glutathione (GSH), and microscale malondialdehyde (MDA) were recorded. Copper/zinc superoxide dismutase (Cu/Zn SOD) is one of the most important free radical scavenging members. To reveal the antioxidative function of <i>P. esculenta,</i> an important member of the antioxidative system, designated <i>Pe-Cu/Zn SOD</i>, was cloned and analyzed. Phylogenic analysis revealed that <i>Pe-Cu/Zn SOD</i> was located in the invertebrate evolutionary branch of intracellular Cu/Zn SOD (icCu/Zn SOD). The quantitative real-time polymerase chain reaction results showed that <i>Pe-Cu/Zn SOD</i> messenger ribonucleic acid was widely expressed in all tissues examined. The highest expression levels in coelomic fluid after Cd exposure indicated its function in the stress response. Using a prokaryotic expression system, we obtained a <i>Pe-Cu/Zn SOD</i> recombinant protein, which enhanced the heavy metal tolerance of <i>Escherichia coli</i>. In vivo assays also confirmed that the <i>Pe</i><i>-Cu/Zn SOD</i> recombinant protein had an antioxidative and free radical scavenging ability. A Cd toxicity experiment, in which purified <i>Pe-</i><i>Cu/Zn SOD</i> protein was injected into the body cavities of <i>P. esculenta</i>, showed that the reactive oxygen species content in the coelomic fluid of the experimental group was significantly lower compared with the control group. These results suggest that <i>Pe</i><i>-Cu/Zn SOD</i> played a role in Cd detoxification by chelating heavy metal ions and scavenging reactive oxygen free radicals, and that <i>P. esculenta</i> could be used as a bioindicator to evaluate heavy metal pollution.