Pesticide combinations may occur in agricultural runoff with each unique mixture of pesticides eliciting adverse effects on aquatic organisms that vary from reported single pesticide responses.  Although pesticide profiles include measures of acute toxicity, limited research has determined the risk of pesticide mixtures to aquatic organisms.  In a 48-h acute bioassay, laboratory organisms were exposed to individual and binary mixtures to compare toxicological endpoints.  The lethal concentration fifty percent (LC₅₀) of lambda cyhalothrin to Pimephales promelas decreased from 2.62 to 2.18 μg/L (p-value, 0.004) when exposed in combination with 100 μg/L of atrazine, and endpoints for Ceriodaphnia dubia decreased from 0.16 to 0.06 μg/L (p-value, 0.022) when exposed in the same combination.  This toxicity study demonstrates that combinations of atrazine and lambda cyhalothrin produce adverse effects when exposed to laboratory test organisms due to potentiation.  Additive, potentiative, or synergistic pesticide interactions may portray detrimental effects to aquatic plants and animals through increased toxicity even at very low concentrations. Therefore, the interactions of pesticides at expected environmental concentrations should be considered in the establishment of risk assessments to aquatic organisms.