The electrogenic C1--pump in the plasmalemma of the marine alga Acetabularia acetabulum (L.) Silva has been suggested to be an unusual type of ATPase (Gradmann, 1989, Methods Enzymol. 174, 490). For a biochemical treatment of this issue, a plasmalemma-rich membrane fraction from Acetabularia has been prepared by phase-partitioning. About 80% of the ATPase activity in this material is inhibited by vanadate (K~50 = 1-2 laM). The phosphohydrolytic properties of the corresponding enzyme were further investigated. Its primary substrate MgATP 2-(Km about 270 gM). Compared with other ptasmalemma ATPases, it has an extremely alkaline pH optimum (pH 8 8.5), a weak sensitivity to diethylstilbestrol and to N,N'-dicyclohexylcarbodiimide, a strong selectivity for Mg 2+ over alternative divalent cations, and a weak selectivity for ATP over other phosphohydrolytic substrates. It is insensitive to KC1 at concentrations up to 200 mM. Both ATP 4-and MgzATP inhibit the ATPase, satisfying a relationship for competitive inhibition by 2ATP 4-(KIATp=l.56mM) and noncompetitive inhibition by Mg2ATP (KiMgzaw= 1.35 mM). Since no transport experiments are reported in this study, the ion species (H + or CI-) that is transferred by this ATPase is not identified.