Daphnia magna Straus were exposed to cadmium for 48 h in water and in equilibrated water-sediment slurries using beakers and recirculating test chambers. Differences in toxicity based on dissolved and free cadmium (Cd2+) between the two levels of tested total solids, 100 and 1000 mg/L, were not significant (α = 0.05). The mean water LC50 of 39 μg/L total cadmium was significantly (α = 0.05) lower than the mean LC50 of 91 μg/L dissolved cadmium in the slurries, indicating that cadmium adsorbed to the sediment had negligible toxicity. Low LC50 values in two sediment bioassays correspond with low pH's. Adsorption isotherms using cadmium, well water, and freshwater sediment were developed for sediment concentrations approximating those at which the bioassays were performed. Conditional adsorption constants derived from the isotherms predicted the distribution of cadmium between well water and sediments. A speciation model predicted that organic ligands released from the sediments strongly complexed the free cadmium ion. Based on free cadmium ion concentrations, no significant differences (α = 0.05) could be detected between the mean of the slurry bioassay LC50 (47 μg/L) and aqueous bioassay LC50 (30 μg/L). Speciation results, LC50 values based on free cadmium ion concentrations only, and pH results gave indirect evidence that the free cadmium ion was the predominant toxic species to D. magna in the bioassays.