The effect of dissolved oxygen concentration on the toxicity of ammonia to juvenile (0.4-3.2 g) gilthead seabream (Sparus aurata) was studied. The mean 96-h LC50 value of ammonia was 23.7 mg/l total ammonia-N (19.3-28.7, 95% C.L.; equivalent to 1.27 mg/l NH3-N). By comparison, S. aurata is somewhat less sensitive to ammonia than salmonids and similar in sensitivity to nonsalmonids. Increased toxicity of ammonia was observed with decreasing oxygen level. Our data suggest that below a threshold of approximately 40% D.O. saturation, the response of S. aurata to ammonia is no longer linear. Under reduced oxygen level most of the mortality occurs within a few hours, apparently as a consequence of an additive toxic effect. No evidence of pathological changes was found in the gills, liver and kidneys of fish with lost equilibrium in the presence of added ammonia. Acutely toxic conditions for S. aurata may prevail in medium-flow systems (2-6 kg fish/m2 and 50% total daily water exchange) where oxygen and ammonia levels are influenced by microbial and algal activities. In such systems, ammonia toxicity may increase as a result of increasing ammonia levels and decreasing oxygen concentration following crashes of phytoplankton populations. In high-flow systems typical of most hatchery and nursery tanks as well as some grow-out ponds, it is likely that the chronic effects of ammonia toxicity will be more important than the acute ones. The data suggest that increasing oxygen level by aeration is sufficient to reduce acute ammonia toxicity under certain situations.