Cytochemical adenosine triphosphatase (ATPase) activity is localised on the plasmalemma and in the tubules of plasmalemmasomes (PLSs) of cells ofChara corallina. Results obtained with centrifuged cells, with only the plasmalemma remaining, tend to confirm that this membrane is the major source of ATPase activity. X-ray microanalysis indicates that ATPase reaction deposits are most likely lead phosphate. ATPase activity is also evident on the chloroplast envelope, the mitochondrial outer membrane, the tonoplast and associated vesicles and commonly throughout the cytoplasm. Results are discussed in relation to the possibilities that the plasmalemma ATPase is a proton pumping ATPase and that the PLS may play a role in enhanced proton efflux.