The concentrations of six trace elements (arsenic, cadmium, copper, lead, mercury, and zinc) were determined in samples of the Pacific oyster,Crassostrea gigas, taken from a retail market and from a local culture area. Concentrations of arsenic, mercury, and particularly cadmium, were frequently greater in samples purchased at the market than in those taken directly from the field. By contrast, levels of copper, lead, and zinc exhibited only minor differences in oysters from the two sources. The differences in the levels of arsenic, cadmium, and mercury are explained in terms of the marketing procedures; in summer, imported oysters dominate the market, and these oysters are relatively more contaminated by these three elements than are locally culturedC. gigas. The implications with respect to public health are discussed and element levels in oysters are compared to both public health regulations in Hong Kong and to metal concentrations in the same species from elsewhere.