The effect of copper on carbohydrate production in the diatom Cylindrotheca fusiformis and in the dinoflagellate Gymnodinium sp. was examined in batch cultures. Copper was shown to inhibit the growth of the two species to different extents: in particular the dinoflagellate was highly sensitive to the metal when metal chelators were absent. Analysis of the carbohydrate content showed that C. fusiformis produced a much higher total amount than Gymnodinium sp. and cells of the diatom which were exposed to toxic copper concentrations displayed a higher carbohydrate production which paralleled the increase in copper concentrations. The increased production could be attributed to extracellular rather than to intracellular carbohydrate and it occurred after 12 and 16 days of growth, respectively, for cells exposed to 0·2 and 0·5 mg l−1 Cu. The response of carbohydrate production by Gymnodinium sp. to the presence of copper was much smaller and occurred only in late stationary phase of growth (21st day). An analysis of the copper content showed that the amount of metal accumulated by Gymnodinium sp. was similar to that absorbed by C. fusiformis, but it was a larger fraction of the quantity supplied. The results presented indicate that carbohydrate extrusion is one of the mechanisms which may prevent toxic effects due to the presence of copper in the culture medium.