Branchial activities of Na(+),K(+)-ATPase (ouabain sensitive), Mg(2+) ATPase (ouabain insensitive) and kinetic analysis of high and low affinity Ca(2+) ATPase were measured inAnguilla anguilla that had been acclimated to demineralized water (DW, Ca < 10 μM), freshwater (FW, Ca = 2 mM), and Low calcium freshwater (L-Ca, Ca = 0.9 mM). Na(+),K(+)-ATPase activity decreased while ouabain insensitive activity increased when ambient Ca(2+) decreased. Two kinetic forms of Ca(2+) ATPase could be resolved in each environmental condition. The stimulation coefficients of both sites or enzymes were not affected by ambient Ca(2+) concentrations. The maximal velocity of both the high and the low affinity Ca(2+) ATPase was increased when external Ca(2+) was decreased during acclimation. The low affinity Ca(2+) ATPase and the Mg(2+) stimulated enzyme could be a non specific enzyme accepting either Ca(2+) or Mg(2+). Results are compared with previous results in the literature and in relation to the branchial morphology and ionic exchanges in fish.