This study explored the effects on Daphnia magna from exposure to the pesticide carbofuran in combination with stress from suspended solids exposure. Our objective was to assess whether suspended solids affects the toxicodynamic response of D. magna to carbofuran. A series of laboratory experiments was performed where animals were exposed to carbofuran concentrations ranging from 0 to 160 mg/l in combination with suspended solids concentrations ranging from 0 to 10,000 mg/l. In the absence of suspended solids, effects of carbofuran were dose dependent and resulted in an EC(sub)50 of 92 mg/l. Exposure to suspended solids, up to extreme levels that may be encountered in the environment and in the absence of carbofuran, showed no measurable toxicity. When D. magna were exposed to a constant carbofuran concentration, the numbers of affected organisms increased with increasing suspended solids concentrations. At a suspended solids concentration of 1,000 mg/l, the EC(sub)50 for carbofuran was reduced by half to 45 mg/l. The relationship between the toxicity of carbofuran EC(sub)50=72 (exp( -0.00014 [suspended solids]). An analysis of the data indicates that this relationship is consistent with a potentiated toxicity mechanism rather than an additive model.