An azide- and vanadate-insensitive, N-ethylmaleimide-sensitive ATPase has been partially purified from a fraction enriched with potassium transporting goblet cell apical membranes of Manduca sexta larval midgut. The properties of the membrane-bound ATPase activity were identical to those of the ATPase activity of highly purified goblet cell apical membranes (Wieczorek, H., Wolfersberger, M. G., Cioffi, M., and Harvey, W. R. (1986) Biochim. Biophys. Acta 857, 271-281). 90% of the azide- and vanadate-insensitive ATPase activity was solubilized by C12E10, leaving 90% of the contaminating azide-sensitive mitochondrial ATPase activity in the pellet after centrifugation at 100,000 x g for 1 h. After discontinuous sucrose gradient centrifugation of the supernatant at 220,000 x g for 1 h nearly all of the azide- and vanadate-insensitive ATPase activity was found in the 30% sucrose fraction without contaminating azide- or vanadate-sensitive ATPase activity. Two prominent bands with relative molecular masses (Mr) of about 600,000 and 900,000, both displaying azide-insensitive and N-ethylmaleimide-sensitive ATPase activity, were found in native microgradient polyacrylamide gel electrophoresis of the 30% sucrose fraction. The two bands could not be separated by anion exchange chromatography. Denaturation of both bands resulted in the same polypeptide pattern (five major bands with Mr 70,000, 57,000, 46,000, 29,000 and 17,000) in sodium dodecylsulfate-polyacrylamide gel electrophoresis, indicating that they represented oligomers of the same protein unit. Substrate and inhibitor specificities of the partially purified ATPase were similar to those of the membrane-bound ATPase activity, whereas salt selectivity differed partly. Altogether, structural and functional properties of the ATPase strongly resemble those of vacuolar-type ATPases.