Ammonia is a metabolite of aquatic organisms which might reach deleterious levels in intensive fish farms. The aim of the present study was to determine median lethal concentrations (96-h LC50) of total ammonia nitrogen (TA-N) on marbled spinefoot rabbitfish (Siganus rivulatus) and chronic effects of TA-N on survival, growth and behaviour of juvenile rabbitfish over a 50 day period. In the first experiment, fish were exposed to 0, 2, 4, 6, 8, 10, 12, 14, 16, 18 and 20 mg L−1 TA-N for 96 h and survival evaluated. In the second experiment, 12 fish were stocked per 50-L tank and treated with one of 0, 2, 4, 6, 8, 10 and 12 mg L−1 TA-N with three replicate tanks per treatment. Survival and growth were determined and histopathological alterations of gills due to chronic ammonia exposure were studied by light and electron microscopy. The 96-h LC50 values were 16–18 mg L−1 TA-N. In the chronic exposure experiment, fish reared in water with 0 mg L−1 TA-N had 100% survival and had 50% weight increase in 50 days. Fish at 2 and 4 mg L−1 TA-N all died whilst fish in 6, 8, 10 and 12 mg L−1 TA-N survived and grew albeit less than in treatment 0 mg L−1. Gills from ammonia treated fish displayed severe histological and ultrastructural alterations including hyperplasia, hypertrophy and fusion of secondary lamellae, aneurysms and presence of pleomorphic altered cells. Chronic exposure to ammonia is deleterious to marbled spinefoot rabbitfish and low concentrations of ammonia appear to kill the fish in <50 days whilst fish can survive for more than 50 days at concentrations between 6 and 12 mg L−1 TA-N.