Branchial chloride cells in the developing larvae and juveniles of freshwater tilapia, Oreochromis mossambicus, were identified and the membrane Na+/K+-ATPase was localized in situ through binding of the fluorescent dye anthroylouabain. After co-labelling of the cells with the fluorescent probes DASPMI and Con-A-FITC, the mitochondria and apical crypt in the same chloride cells were visualized using confocal laser scanning microscopy. The high density of apical crypts indicated that many chloride cells were functional. The density of branchial chloride cells in larvae 10 days after hatching was approximately 6000 mm-2. An extremely high Na+/K+-ATPase specific activity of approximately 1500 µmol Pi h-1 mg-1 was measured in the gills 10 days after hatching. With the development of secondary lamellae and hence an increase in the amount of branchial epithelial protein, a concomitant decrease in the specific activity of the enzyme in the gill tissues was observed. Total Na+/K+-ATPase activity increased markedly in the early life stages. Our data indicate that in larval stages of fish the gills form a functional ionoregulatory organ before they start functioning as a gas-exchange organ.