In the natural environment, heavy metal contamination can occur as long-term pollution of sites or as pulses of pollutants from wastewater disposal. In this study two freshwater green algae, Chlorella sp. FleB1 and Scenedesmus YaA6, were isolated from lead-polluted water samples and the effects of 24 h vs 4 and 8 d exposure of cultures to lead on growth, photosynthetic physiology and production of reactive oxygen species (ROS) of these algae were investigated. In Chlorella sp. FleB1, there was agreement between lead impacts on chlorophyll content, photosynthesis and growth in most case. However, in Scenedesmus acutus YaA6 growth was inhibited at lower lead concentrations (0.03-0.87 × 10-9 M), under which ROS, measured by 2',7' dichlorodihydrofluorescein diacetate fluorescence, were 4.5 fold higher than in controls but photosynthesis was not affected, implying that ROS had played a role in the growth inhibition that did not involve direct effects on photosynthesis. Effects of short-term (5 h, 24 h) vs long-term (4 d and 8 d) exposure to lead were also compared between the two algae. The results contribute to our understanding of the mechanisms of lead toxicity to algae.