The growth rate and maximum yield of Skeletonema costatum (Grev.) Cleve in batch culture were inhibited by dissolved lead at concentrations of 0.05 to 10.0 μg Pb l-1. Growth rate, maximum yield, and respiration cell-1 decreased and photosynthesis cell-1 and cell volume increased in response to increased lead concentration in the medium. At 0.1 and 1.0 μg Pb l-1, the chlorophyll a:carbon and protein:carbon ratios did not significantly differ from the control and the increased cellular chlorophyll a, carbon and protein observed at these concentrations reflect an increased cell volume. However, at 10.0 μg Pb l-1, associated with an increase in cell volume, the chlorophyll a:carbon ratio was significantly lower and the protein:carbon ratio was significantly higher than in the control, reflecting a changed cellular chemical composition. The rates of cell division and dark respiration decreased relative to photosynthesis and to carbon and protein production, resulting in an alteration of the cellular chemical composition.