The marine diatom Phaeodactylum tricornutum Bohlin was exposed to different cadmium concentrations (1-100 mg l(-1)) for 4 days. The amount of cadmium removed was recorded, with particular attention paid to long-term uptake kinetics, and to the cellular location of cadmium. Cadmium accumulation occurred at all concentrations assayed. The EC50 of cadmium to P. tricornutum was 22.39 mg l(-1) after 4 days of exposure. Cadmium uptake followed a saturation kinetic at cadmium concentrations greater than or equal to 25 mg l(-1). However, at lower cadmium concentrations, the uptake of this metal followed a linear trend for all days of culture. At cadmium concentrations in the medium lower than 25 mg l(-1), P. tricornutum removed cadmium mainly within the cell. At higher cadmium concentrations, the amount of cadmium removed by adsorption to the cell surface was higher than intracellular cadmium, because of the toxic effects of cadmium on P. tricornutum cells. This toxicity reduced the cadmium accumulation within the cells.