Abstract: The toxicity of mercuric chloride (as Hg++) to Daphnia magna was studied under renewed static conditions. Under 25±℃ and 9—10 hour dialy lighting (3,000—3,300 lux), Daphnia magna, derived from a pure strain and aged 6±6 hours, were exposed to 1—28μg Hg/L. Experiment of each concentration was carried out by means of ten beakers, each containing 50ml of filtered water and a single Daphnia. Scenedesmus obliquus was provided for the animals throughout the experiments.The LC50 of mercuric chloride to D. magna was estimated to be 13.5±2.1μg/L for 48 hours. The chronic test showed that Daphnia died totally in 3.40±0.84 days when the mercuric concentration increased μp to 28μg/L, but there were no significant differences in the survival rates and growth rates in the media with concentrations ranging from 1 to 21μg Hg/L except for the totalnumber of offspring. In addition, the net reproductive rate (R0) of the animals was found to decline gradually with increasing mercuric concentration.