Plasma membrane (PM) H(+)-ATPase and H(+) transport activity were detected in PM fractions prepared from Zostera marina (a seagrass), Vallisneria gigantea (a freshwater grass) and Oryza sativa (rice, a terrestrial plant). The properties of Z. marina PM H(+)-ATPase, specifically, the optimal pH for ATPase activity and the result of trypsin treatment, were similar to those of authentic PM H(+)-ATPases in higher plants. In V. gigantea and O. sativa PM fractions, vanadate-sensitive (P-type) ATPase activities were inhibited by the addition of NaCl. In contrast, activity in the Z. marina PM fraction was not inhibited. The nitrate-sensitive (V-type) and azide-sensitive (F-type) ATPase activities in the Z. marina crude microsomal fraction and the cytoplasmic phosphoenolpyruvate carboxylase activity, however, were inhibited by NaCl, indicating that not all enzyme activities in Z. marina are insensitive to salt. Although the ratio of Na(+) to K(+) (Na(+)/K(+)) in seawater is about 30, Na(+)/K(+) in the Z. marina cells was about 1.0. The salt-tolerant ATPase activity in the plasma membrane must play an important role in maintaining a low Na(+) concentration in the seagrass cells.