As part of an effort to understand extensive, white phosphorus (P₄)-induced waterfowl mortality at Eagle River Flats, Fort Richardson, Alaska (USA), we conducted a number of acute toxicity tests using penned mallards (Anas platyrhynchos) in 1993 and 1994. The 24-hr median lethal dose (LD₅₀) for P₄ dissolved in oil was 6.46 mg/kg in adult males and 6.96 mg/kg in adult females. Although the median lethal doses were not statistically different, the female dose-response curve had a statistically shallower slope than that of males. The LD₅₀ for the ecologically more relevant pelletized form of P₄ in adult males was 4.05 mg/kg. In mallards, one mechanism of P₄ toxicity caused rapid (3 to 10 hr) mortality and had signs consistent with anoxia. A second, slower acting mechanism resulted in hepatic and renal pathology including extensive fat deposition in the liver and cellular necrosis. White phosphorus accumulated in adipose tissues, but only for a few days.