This is the first study concerning the effects of salinity on the gill ion secreting mechanisms of the blackchinned tilapia Sarotherodon melanotheron heudelotii originating from a hypersaline estuary (Saloum, Senegal). Under controlled conditions, juvenile fish initially kept in freshwater (FW) were divided into two groups exposed to FW or directly to 35 psu seawater (SW). Part of SW fish were then acclimatized to hypersaline waters (HW) by a step-wise increase of 7 psu/day up to 70 (HW70) and 90 psu (HW90). Three major ion transporters in gill ionocytes, Na+/K+-ATPase, the cotransporter Na+/K+/2Cl? (NKCC) and Cl? channel (CFTR) were localized by immunofluorescence and ionocyte localization, number and size were investigated. In FW, ionocytes immunostained with Na+/K+-ATPase were only observed at the base of the filaments, whereas in SW and particularly in HW they were localized on the filaments and expanded extensively to the lamellae, indicating the functional challenge that these fish encounter. A significant increase in ionocyte number and size (area) occurred following transfer from FW to SW and HW. Although the cell number remained constant between HW70 and HW90, their size increased from FW to HW90. The marked increase in cell number and size indicates that the cells are functioning at a high intensity in HW. In FW, Na+/K+-ATPase was concentrated at the basolateral region while NKCC staining was apical and CFTR was not observed. In SW and HW, Na+/K+-ATPase and the cotransporter NKCC were colocated on the basolateral membrane, while CFTR was located at the apical region. These findings agree with the literature on other chloride secreting cells, in which a relation between the location of the transporters and chloride secretion could be established.