Ability to surpass osmoregulatory challenges posed by environmental salinity can be a key factor determining the colonizing success of invasive species. The neotropical cichlid Australoheros facetus , is invasive upstream the Guadiana and Arade estuaries in southern Portugal, where flash floods occur during the rainy season and could drag fish downstream. This work aimed at ascertain if this species can colonize the upper estuary, looking at physiological and behavioural responses of fish reared in a range of salinities. Set 1: fish (4-6cm, 2xN=30) were reared for 90 days at 0-6-12-18ppt, weighted every 15 days and sampled every 30 days. Set 2: fish (9-11cm, 3xN=60) were subject to an increase of 3ppt each 3 days. After 5 days of acclimation at 0-6-12-18ppt, blood and tissues were collected for osmoregulatory parameters and four social groups (5 fish/group at 0-6-12ppt) were formed and observed for one week for behavioural analyses. In set 1 growth reduction was obvious after 30 days of experiment at 18ppt when compared with other groups (p<0.05). Aggressive behaviour was also absent and mortality reach 56% in this group. In set 2 survival rate was only 25% after 5 days at 18ppt. Muscle water content, plasma lactate and total protein showed a significant decrease, while plasma osmolality, chloride and glucose increased at 18ppt in relation to control group. For the other groups we did not observe salinity-related differences in plasma substrates or electrolytes (glucose, lactate and chloride), aggressive interactions, territorial status or dominance index. In salinities above the isosmotic point there was an increase in the activity of branchial Na/K-ATPase, an energy demand for the maintenance of hydromineral balance which may result in reduced growth and activity. Such conditions may lead to the exhaustion phase of the stress syndrome, not allowing this species to survive for long periods in estuarine environments.