Although it has been well established that different species of marine algae have different sensitivities to metals, our understanding of the physiological and biochemical basis for these differences is limited. This study investigated copper adsorption and internalisation in three algal species with differing sensitivities to copper. The diatom Phaeodactylum tricornutum was particularly sensitive to copper, with a 72-h IC50 (concentration of copper to inhibit growth rate by 50%) of 8.0 μg Cu L-1, compared to the green algae Tetraselmis sp. (72-h IC50 47 μg Cu L-1) and Dunaliella tertiolecta (72-h IC50 530 μg Cu L-1). At these IC50 concentrations, Tetraselmis sp. had much higher intracellular copper (1.97 ± 0.01 x 10-13 g Cu cell-1) than P. tricornutum (0.23 ± 0.19 x 10-13 g Cu cell-1) and D. tertiolecta (0.59 ± 0.05 x 10-13 g Cu cell-1), suggesting that Tetraselmis sp. effectively detoxifies copper within the cell. By contrast, at the same external copper concentration (50 μg L-1), D. tertiolecta appears to better exclude copper than Tetraselmis sp. by having a slower copper internalization rate and lower internal copper concentrations at equivalent extracellular concentrations. The results suggest that the use of internal copper concentrations and net uptake rates alone cannot explain differences in species sensitivity for different algal species. Model prediction of copper toxicity to marine biota and understanding fundamental differences in speciessensitivity will require, not just an understanding of water quality parameters and copper-cell binding, but also further knowledge of cellular detoxification mechanisms.