Euryhaline fishes such as the barramundi (Lates calcarifer), have the physiological ability to move between waters of various salinities and are often exposed to rapid changes in ionic gradients that may alter acid-base homeostasis. It has recently been proposed that these species may utilize gill H +ATPase and Na +/H + exchanger mechanisms to regulate systemic pH, and that the differential expression of the acid/base relevant transporters is governed by environmental salinity. This study examined acid-base regulation in juvenile barramundi adapted to fresh, brackish, or salt water (ranging from 0 to 32ppt) and exposed to 1% environmental hypercapnia. Our findings suggest that barramundi increase net acid excretion in all salinities tested, with a more rapid response observed in fish adapted 22,5 and 32ppt. In contrast, the relative change in net H + transfers was greatest in the groups adapted to the lower salinities. Fish in 15ppt exhibited an apparent relationship between increased in net H + excretion and increases in branchial H +ATPase protein expression. © 2012 Elsevier B.V.