The mudskipperB. boddaerti, was able to survive in waters of intermediate salinities (4-27‰). Fish submerged in dechlorinated tap water suffered 60% mortality by the fifth day while 60% of those in 100% sea-water (sw) died after the third day of exposure. After being submerged in 50% or 80% sw for 7 days, the plasma osmolality, plasma Na(+) and Cl(-) concentrations and the branchial Na(+) and K(+) activated adenosine triphosphatase (Na(+),K(+)-ATPase) activity were significantly higher than those of fish submerged in 10% sw for the same period. However, the activities of the branchial HCO3 (-) and Cl(-) stimulated adenosine triphosphatase (HCO3 (-),Cl(-)-ATPase) and carbonic anhydrase of the latter fish were significantly greater than those of the former. Such correlation suggests that Na(+),K(+)-ATPase is important for hyperosmotic adaptation in this fish while HCO3 (-)-Cl(-)-ATPase and carbonic anhydrase may be involved in hypoosmotic survival.