The tolerance of the European sea bass to high salinities was determined through abrupt and gradual transfers to different hyperosmotic media. Mortality rate reached roughly 50% in fish abruptly transferred to 70 psu salinity and in fish gradually adapted to 90 psu salinity. All fish abruptly transferred to 90 psu salinity expired in less than three hours. A crisis and regulation type osmoregulatory pattern, related to the range of the osmotic shock, was recorded following abrupt transfers to the tolerable experimental media. The lethal level of blood osmolality in sea bass was determined.