Toxic effect of Zn(II) on a green alga (Chlorella pyrenoidosa) in the presence of sepiolite and kaolinite was investigated. The Zn-free clays were found to have a negative impact on the growth of C. pyrenoidosa in comparison with control samples (without adding any clay or Zn(II)). When Zn(II) was added, the algae in the presence of clays could be better survived than the control samples, which was actually caused by a decrease in Zn(II) concentration in the solution owing to the adsorption of Zn(II) on the clays. When the solution system was diluted, the growth of algae could be further inhibited as compared to that in a system which had the same initial Zn(II) concentration as in the diluted system. This in fact resulted from desorption of Zn(II) from the zinc-contaminated clays, although the effect varied according to the different desorption capabilities of sepiolite and kaolinite. Therefore the adsorption and desorption processes of Zn(II) played an important part in its toxicity, and adsorption and desorption of pollutants on soils/sediments should be well considered in natural eco-environmental systems before their risk of toxicity to aquatic organisms was assessed objectively.