The toxicity of leachate water from acid-sulphate soil to the early life stages of Australian bass, Macquaria novemaculeata, incubated in seawater was evaluated. Acid-sulphate soil leachate water (pH ≤ 6.8) delayed the hatching of fertilised eggs, but after 48 h the per cent hatching was normal. In comparison, acidic saline water (25‰ salinity) at pH 4.0 or less prevented embryos from hatching. The survival of yolk-sac larvae exposed to acid-sulphate soil leachate water at a concentration of 32% in seawater and an initial pH of 7.2, was significantly different to controls after 96 hours. In corresponding tests with only acidified saline water (20‰ salinity), pH levels equal to or below 5.0 killed yolk-sac larvae after 96 h exposure. Aluminium showed a pH dependent toxicity to yolk-sac larvae, with added aluminium as low as 200 μg litre-1 having a significant effect on larval survival at pH 5.5, and concentrations of 600-800 μg litre-1 having a significant effect on larval survival at an initial pH range of 6.0 < pH < 6.8. It was concluded that significant mortality of the early life stages of Australian bass would occur if they are exposed to acid-sulphate soil leachate that results in a pH in the receiving estuarine water below 5.5, or when the pH is below 6.8 and aluminium is present at a total concentration of 800 μg litre-1 or greater.