Abstract: In order to compare the chronic toxicity of pollutants to organisms in an aquatic environment under different food densities, the cladoceran Moina macrocopa was used as test animal to investigate the effects of Hg²⁺ concentrations (0, 0.4, 1.1, 1.8, 2.5, 3.2 and 3.9 μg·L^-1) on its life table demography under low (0.5×10⁶ cells·mL^-1), medium (1.0×10⁶ cells·mL^-1) and high (2.0×10⁶ cells·mL^-1) densities of Scenedesmus obliquus. The results showed that at low and high food levels, certain concentrations of Hg²⁺ had inhibitory effects on survival, reproduction and population growth of M. macrocopa. However, at the medium food level, the inhibitory effects disappeared. Compared with the control, at the low food level, Hg²⁺ at 0.4-2.5 and 3.9 μg·L^-1 significantly decreased the life expectancy at hatching, Hg²⁺ at 0.4, 1.1, 2.5 and 3.9 μg·L^-1 decreased the generation time, Hg²⁺ at 1.1, 2.5 and 3.9 μg·L^-1 decreased the net reproduction rate, and Hg²⁺ at 3.9 μg·L^-1 decreased the gross reproduction rate of M. macrocopa. At the high food level, Hg²⁺ at 1.1 μg·L^-1 decreased the intrinsic rate of population increase, and Hg²⁺ at 3.9 μg·L^-1 decreased the life expectancy at hatching and the net reproduction rate of M. macrocopa. However, at the medium food level, Hg²⁺ at 3.2 μg·L^-1 increased the life expectancy at hatching of M. macrocopa. When S. obliquus density was 0.5×10⁶ cells·mL^-1, there were significant doseeffect relationships between Hg^2+ concentration and the life expectancy at hatching, the generation time as well as the cross reproduction rate of M. macrocopa. However, when S. obliquus density was 1.0×10⁶ or 2.0×10⁶ cells·mL^-1, there were no any significant doseeffect relationships between Hg²⁺ concentration and each of the life table demographic parameters of M. macrocopa.