The toxicity of leachate water from acid-sulfate soil to the early embryonic development of the Sydney Rock oyster, Saccostrea commercialis, was assessed. Concentrations of acid-sulfate soil leachate water as low as 3.3% in seawater were found to decrease the normal development of oyster embryos after 48 hr exposure, and this effect could not be attributed to any significant change in pH or salinity. An EC50 value for the acid-sulfate soil leachate water of 2.5 to 2.9% in seawater was obtained, and the no observed effect concentration was determined at a concentration of 2% in seawater. In tests conducted with aluminum added to seawater, a significant decrease in the percentage of embryos developed to the D-veliger stage occurred at concentrations of 150 μg/liter and greater, with no effects at 100 μg/liter. An EC50 of 225 μg/liter for the effect of added aluminum on embryo survival was obtained and all embryos showed developmental abnormalities at concentrations of 400 μg/liter and greater. A significant decrease in the embryonic development occurred when the fertilized eggs were incubated in pH-adjusted seawater at pH values ≤6.75, but no significant effects were found at pH 7.0 or above. Since aluminum was present in high concentrations in the acid-sulfate soil leachate water, it was concluded that aluminum was the main toxicant in the arid-sulfate water that disrupted the oyster embryonic development.