Abstract: The effects of aquatic humic acids on the bioconcentration and acute toxicity of meothrin were evaluated using a freshwater grass carp, Ctenopharyngodon idellus in laboratory freshwater systems. The results demonstrated that both the bioavailability and acute toxicity were decreased significantly by the presence of aquatic humic acid at the concentration of 5 and 10mg/L. In addition, the extents of the influences were increased with increasing concentration of aquatic humic acids.