Cadmium toxicity to the diatom Phaeodactylum tricornutum Bohlin has been studied on the basis of the effect of this metal on growth, ATP content, 14C uptake and cellular ultrastructure. The microalga was exposed to 1, 5, 10, 25, 50, 75 and 100 mg 1−1 of cadmium and showed a concentration-dependent inhibition in those physiological parameters. At cadmium concentrations of 5 mg 1−1 or higher, a significant effect on growth of P. tricornutum was observed. After 8 hours of exposure to the different cadmium concentrations, the decrease in the ATP content was significant only at cadmium concentrations of 25 mg 1−1 or higher. 14C uptake was more susceptible to cadmium than the decrease in the ATP content, as 5 mg 1−1 of cadmium caused a severe decrease in this parameter. Cadmium also caused ultrastructural changes in P. tricornutum cells: deposition of cadmium on the surface of cell, increase in the chloroplast size, appearance of electrodense granulations, and reduction in lipid inclusions.