The effect of different sources of dissolved humic material on the acute toxicity of the pyrethroid insecticides fenvalerate (Pydrin®) and permethrin (Ambush®) was examined by using the Microtox® bioassay. Toxicity of the insecticides was examined in the presence of commercially available humic acid (HA) as well as aquatic HA and fulvic acid (FA) at concentrations of 0, 0.5, 5.0, 50, and 100 mg/L. The toxicity of fenvalerate was significantly decreased in the presence of 50 and 100 mg/L of commercially available HA and 100 mg/L of aquatic HA. Aquatic FA had no significant influence on the toxicity of fenvalerate. For permethrin, toxicity was significantly decreased at 50 and 100 mg/L of commercially available HA and at all examined concentrations of aquatic HA and FA. However, the magnitude of toxicity amelioration by aquatic HA and FA was not as great as it was with commercially available HA. These results indicate that interactions between HA or FA and pyrethroid insecticides can alter the toxicity of pyrethroid insecticides. Furthermore, the extent of alteration was dependent on the source of dissolved humic material (DHM).