Since Na$$K$-activated ATPase was first described, using arthropod tissues, it has become well-recognized as the enzymatic equivalent of the sodium pump. Occurring in the basolateral plasma membranes of epithelial ion-transporting cells, it is responsible for the transport of Na$ out of cells in exchange for the cytosol-directed movement of a counterion (K$ or NH4$). Its kinetic and dynamic properties suggest that it serves as a major limiting factor in whole-body Na$ regulation by aquatic arthropods. Its contribution to NH4$ excretion awaits isolation of Na$ $ K$-ATPase-enriched membrane fractions and determination of their transport properties. The role of Na$$K$-ATPase in insect epitheha is made uncertain by the apparent inaccessibility of the ATPase to the inhibitor ouabain. Two other membrane-bound ATPases, K$-stimulated ATPase and anion-dependent ATPase, have been described in arthropod tissues, but their physiological roles are not clear.