Daphnia magna Straus were exposed to cadmium for 48 h in water and in equilibrated water sediment slurries using breakers and recirculating test chambers. Differences in toxicity based on dissolved and free cadmium (Cd(2+)) between the two levels of tested total solids 100 and 1000 mg/l, were not significant. The mean water LC50 of 39 micrograms/l total cadmium was significantly lower than the mean LC50 of 91 micrograms/l dissolved cadmium in the slurries, indicating that the cadmium adsorbed to the sediment had negligible toxicity. Low LC50 values in two sediment bioassays correspond with low pHs. Adsorption isotherms using cadmium, well water, and freshwater sediment were developed for sediment concentrations approximating those at which the bioassays were performed. 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.