The brush border membrane of the surface epithelial syncytium of the parasitic flatworm, Hymenolepis diminuta, was isolated by saponin treatment, and the ATPase activity was determined. The ATPase activity was insensitive to ouabain, SCN−, HCO3−, ethanol, and, although slightly inhibited by high oligomycin concentrations, was inhibited by ethacrynic acid. Reaction mixtures made increasingly hypertonic with mannitol resulted in vesicular changes in the microvilli and decreased ATPase activity. Although treatment of the brush border fraction with deoxycholate increased ATPase activity, the dependence of activity on the osmotic pressure of the reaction mixture persisted in the solubilized membrane preparations. ATPase activity was dependent upon Mg2+ and Na+ + K+ concentration. It is suggested that the ATPase in the flatworm surface membrane is involved in volume regulation in hypotonic media.