Non-native freshwater fishes may spread via estuaries, invading nearby basins. The Brazilian natives Rhamdia quelen and Geophagus brasiliensis, and the worldwide introduced Clarias gariepinus, Ictalurus punctatus, Oreochromis niloticus and Cyprinus carpio were acutely exposed (6 h) to salinities 15 and 30. Hypothetically, the introduced species display greater physiological plasticity than do the natives. Exposure to salinity 30 was lethal after 1.5–3 h to all species except for O. niloticus and G. brasiliensis. Increase in plasma osmolality was inversely related to muscle water content, mainly in salinity 30 for all species. R. quelen and C. gariepinus displayed increased expression of heat-shock protein 70 (HSP70) on salinity increase; differently, I. punctatus, O. niloticus and C. carpio showed high constitutive levels already in freshwater. Species with high constitutive expression of HSP and/or high degree of euryhalinity (cichlids) could potentially use estuaries as bridges, especially through areas of salinity <15.