The current study investigated acute toxicity to ammonia of the South African abalone, Haliotis midae, from three size classes relevant to mariculture operations, and the chronic impact of sub-lethal ammonia levels on growth of juvenile abalone.Results showed that tolerance to ammonia (at pH 7.8 and T-a = 15 degrees C) increases with body size (i.e. age) as indicated by 36 h LC50 values: juvenile abalone (1-2.5 cm shell length) had the lowest LC50 of 9-8 mu g l(-1) FAN, whereas LC50 was 12.9 mu g l(-1) FAN in "cocktail"-size abalone (5-8 cm shell length). The highest LC50 of 16.4 mu g l(-1) FAN was observed in "brood stock"-size animals (10-15 cm). When "cocktail"-size abalone were allowed to acclimatize to sub-lethal ammonia levels for 48 h, their ammonia tolerance increased compared with non-acclimatized abalone of the same size: LC50 was 2.0 mu g l(-1) FAN higher at 14.8 mu g l(-1) FAN.Growth of juvenile abalone (1-2.5 cm shell length) during chronic exposure to sub-lethal FAN levels is inhibited: specific growth rate (SGR) is significantly reduced by 58.7% to 0.10 +/- 0.03% d(-1) (weight) compared with 0.24 +/- 0.06% d(-1) of abalone of a control group (no ammonia).The results demonstrate the negative effects of ammonia not only on survival but also on growth of farmed abalone, both impair profitability of the fanning operation. The information from the present study will be helpful in determining water quality requirements in South African abalone farms. (c) 2006 Elsevier B.V. All rights reserved.