[en] Biological uptake of Pu was investigated using two common freshwater diatoms species, Asterionella formosa and Fragilaria crotonensis, which were exposed to both oxidized (V + VI) and reduced (III + IV) plutonium. Results indicate that although plutonium in both oxidation states distributes throughout the cell, the oxidized form appears to cross the cell wall more rapidly than the reduced form. The plutonium found within the cells subsequent to the administration of the oxidized form was largely reduced, suggesting that cells may act as a reducing agent for plutonium in nature