AbstractDimensions and area densities of membrane particles were studied by electron microscopy of replicas of freeze‐fractured suspensions of tonoplast vesicles ofMesembryanthemum crystallinumL. in the C‐3 state and after induction of crassulacean acid metabolism (CAM) by salinity. The results are compared with the relative contribution of tonoplast‐ATPase protein to total membrane protein obtained from integration of elution profiles in size‐exclusion chromatography. Coverage of tonoplast area by globular membrane particles was 20% and 36 % and ATPase in relation to total membrane protein was 33 % and 35 % in C‐3 and CAMM. crystallinum, respectively. Thus, by order of magnitude, it is most likely that the particles represent the ATPase. InKalanchoë daigremontianaHamet et Perrier de la Bâthie the ATPase also constituted 36% of total tonoplast protein. Induction of CAM inM. crystallinumwas associated with an increase in specific ATPase activity of the tonoplast and an increase of the size and area coverage of tonoplast particles, whereas the relative contribution of ATPase protein to total tonoplast protein and the molecular mass of the ATPase holoenzyme, as seen in size‐exclusion chromatography, remained alm