The speciation and bioavailability of metals are known to be affected by pH. Although many studies have focused on effects on metals of pH changes resulting from lake acidification (i.e., pH change 7 to 4), metal toxicity changes at higher pH values (i.e., pH 6-9) are of great interest to those performing effluent and sediment toxicity testing and toxicity identification evaluations (TIEs). In addition, most previous studies have addressed metal toxicity changes with pH to water-column organisms rather than to benthic or epibenthic species. The authors tested the acute toxicity of five metals (Cd, Cu, Ni, Pb, and Zn) to Ceriodaphnia dubia, Pimephales promelas, Hyalella azteca, and Lumbriculus variegatus at three pH values (6.3, 7.3, and 8.3) in very hard reconstituted water (hardness 300-320 mg/l as CaCO3). Toxicity of Cd, Ni, and Zn was greatest at pH 8.3 and least at pH 6.3 to most of these species. Conversely, the toxicity of Cu and Pb was greatest at pH 6.3 and least at pH 8.3 to most of the species.