Snakeheads were adapted to fresh water (1 mOsm kg−1), 25% sea water (230 mOsm kg−1), 33% sea water (320 mOsm kg−1) and 40% sea water (380 mOsm kg−1) for 20 days. Exposure to salt water resulted in tissue dehydration, elevations of plasma osmolality, Na+, Mg2+, Cl−and protein concentrations and stimulation of branchial Na+‐K+‐ATPase activity. These changes were accompanied by concomitant decline of the hepatosomatic index and liver glycogen concentration. The routine rate of oxygen consumption was increased in snake‐heads adapted to 33% sea water. These data were taken to indicate a stressful effect of salinity to the snakehead despite documentation of its ability to penetrate into