[en] Sensitivity of the Pacific oyster, Crassostrea gigas, to copper was evaluated in experiments of 2-wk duration. Groups of 8 to 10 animals were exposed to copper concentrations ranging from 100 to 1300 μg/1 in a high-volume, flow-through bioassay system. The greatest mortality at 96 h occurred in animals at intermediate rather than high copper concentrations. However, by correcting the duration of exposure of each oyster to copper by a factor related to the amount of time the shells were open, we obtained typical mortality curves. Significant mortalities occurred at copper concentrations greater than or equal to 200 μg/1. Distribution of ⁶⁴Cu in oysters after 24-h exposure differed between animals maintained in water containing high and low concentrations of copper. Copper flux into tissues increased with the copper concentrations in the water, indicating little metabolic regulation