Changes in branchial Na+-K+-ATPase activity, and the number and size of chloride cells resulting from the transfer of fish into seawater were investigated in tilapia (Oreochromis niloticus) (22.03 +/- 0.91 g), which were transferred to full strength seawater (36 parts per thousand) directly and for 14 days. Whole mortality occurred when the tilapia were transferred into seawater directly. That is, no acclimation was allowed. Branchial chloride cell numbers decreased after seawater exposure, whereas a gradual increase was observed in chloride cell sizes. However, the chloride cells of seawater-adapted individuals showed a 2-fold increase in size (P < 0.05). Initially 5 parts per thousand and 10 parts per thousand salinity resulted in lowered branchial Na+-K+-ATPase activity but then this activity increased and the highest activity was at 25 parts per thousand salinity (P < 0.01). This study demonstrated the effects of high salinity through direct and gradual acclimations on branchial Na+-K+-ATPase activity and chloride cell abundance.