Diatoms are vital primary producers in marine ecosystems, and their growth may be affected by harmful concentrations of zinc in the environment. However, limited information exists on the response of some important physiological parameters to zinc as contaminant. In the present study, we evaluated the effect of zinc exposure on growth, chlorophyll a and in vivo fluorescence of the diatom Conticribra weissflogii under laboratory conditions. Cell numbers were significantly reduced after 48 h exposure to 450 and 900 μg Zn L-1. Growth rates estimated after 96 h exposure (in div day-1) were 0.81, 0.85, 0.69 and 0.59 for cells exposed to control, 90, 450 and 900 μg Zn L-1, respectively, and it was significantly reduced at 900 μg Zn L-1. There was a tendency of increase in chlorophyll a per cell after 3 h exposure to zinc. However, the ratio of in vivo fluorescence to chlorophyll a strongly decreased after 3 h exposure to zinc, suggesting that the mechanism of toxicity would involve a zinc-induced damage to the photosystem II. Possible metabolic routes for this damage and its environmental implications are discussed.