Abstract 1. 1. Bass gill microsomal preparations contain a Mg 2+-dependent Na +-stimulated ATPase activity in the absence of K +, whose characteristics are compared with those of the (Na + + K +)-ATPase of the same preparations. The activity at 30°C is 11.3 μmol Pi-mg −1protein-hr −1 under optimal conditions (5mM MgATP, 75 mM Na + 75 mM HEPES, pH 6.0) and exhibits a lower pH optimum than the (Na + + K +-ATPase. 2. 2. The Na + stimulation of ATPase is only 17% inhibited by 10 −3M ouabain and completely abolished by 2.5mM ethacrinic acid which on the contrary cause, respectively, 100% and 34% inhibition of the (Na + + K +)-ATPase. 3. 3. Both Na +- and (Na + + K +) stimulated activities can hydrolyze nucleotides other than ATP in the efficiency order ATP > CTP > UTP > GTP and ATP > CTP > GPT > UTP, respectively. 4. 4. In the presence of 10 −3M ouabain millimolar concentrations of K + ion lower the Na + activation (90% inhibition at 40 mM K +). 5. 5. The Na +-ATPase is less sensitive than (Na + + K +)-ATPase to the Ca 2+ induced inhibition as the former is only 57.5% inhibited by a concentration of 1 × 10 −2M which completely suppresses the latter. 6. 6. The thermosensitivity follows the order Mg 2+- > (Na + + K +)- > Na +-ATPase. A similar break of the Arrhenius plot of the three enzymes is found. 7. 7. Only some of these characteristics do coincide with those of a Na +-ATPase described elsewhere. A presumptive physiological role of Na +-ATPase activity in seawater adapted teleost gills is suggested.