The effect of salinity on survival, growth, haematological parameters and osmoregulation was evaluated in tambaqui. This study addressed two questions. Firstly, tambaqui (33.9 AE 3.4 g) reared in freshwater were subjected to gradual increases in salinity until 100% mortality. Two daily increments of salinity were tested with three replicates: 1 and 2 g L À1 day À1 , while control group was kept in freshwater. For 1 g L À1 day À1 , the first mortality appeared from a salinity of 11 g L À1 and the last dead fish was observed at a salinity of 20 g L À1 , whereas for 2 g L À1 day À1 , the mortalities occurred only between 20 and 22 g L À1 . The main sub-lethal effects observed were food intake stopped, erratic swimming behaviour, increased mucus production and the dark pattern of pigmentation that appeared at 13 and 16 g L À1 for both treatments. Secondly, juveniles (57.4 AE 6.3 g) were reared at 0, 5, 10 and 15 g L À1 , in triplicate, for 84 days. At the end of the experiment, the results indicated that the survival, growth, haematological parameters and osmoregulatory of fish were significantly affected by salinity. Survival was unaffected by 10 g L À1 , whereas higher salinity (15 g L À1 ) had a detrimental effect. Differences in final weight, specific growth rates (SGR) and mean daily feed intake (MDFI) among treatments were significant. There were no significant trends in feed conversion rate (FCR) at salinities of 0, 5 and 10 ppt. In these salinities, FCR ranged between 1.2 AE 0.1 and 1.3 AE 0.1, whereas FCR at salinity of 15 g L À1 increased to 5.8 AE 4.4. At the end of the experiments, analysis of blood revealed that all haematological parameters were affected by increase in salinity. From the present investigation, it can be concluded that exposure to salinity significantly affects the survival, growth and physiological response of tambaqui.