CADMIUM is a major environmental pollutant potentially harmful to health, and if the sea becomes polluted with this metal there could be a reduction in extensive sources of food1. Seafoods constitute a source of cadmium in the human diet2 and in view of abundant evidence that shellfish accumulate trace metals3–6, it is important to investigate cadmium pollution. People have become ill from cadmium poisoning after ingesting foods containing concentrations of 13–15 µg g−1 (13–15 p.p.m., ref. 7). We now report that adult oysters reared in seawater containing 0.005 p.p.m. cadmium accumulated up to 10.75 p.p.m. in 40 weeks. This accumulation, plus cadmium naturally present, brought the concentration of cadmium to 13 p.p.m. in the soft tissue, which represents a potential health hazard if oysters constitute a major item of the diet.