The acute toxicities of both organic flocculants (polymer) and inorganic coagulant aids (Al-2(SO4)(3) and FeCl3) to the commonly used whole effluent toxicity test species Ceriodaphnia dubia were evaluated. Results from these studies suggested that cationic polymer flocculants have the potential of inducing acute toxicity to C. dubia at extremely low concentrations as opposed to the inorganic coagulant aids. Because polymers and inorganic coagulant aids are more commonly used in conjunction with one another, assessment of potential toxic interaction between the two types of additives was evaluated, as well, using joint-compound interaction modeling. Results from these studies indicated that cationic polymers and FeCl3 or Al-2(SO4)(3) seemed to act synergistically (greater than additive toxicity). Overall, these studies clearly demonstrated that both the type of flocculating agent used and the type of treatment application strongly influence toxicity.