Changes in δ13C value, diurnal malate content, water content and Na+, K+ and Cl- content of the annual Mesembryanthemum crystallinum (Aizoaceae) were followed in a natural population on a coastal cliff at the Mediterranean Sea shore close to Caesarea (Israel). Plants germinated in the middle of the rainy season in December 1976/January 1977. Diurnal malate fluctuations in the leaves were not detected until the end of March. Later on, at the start of the dry season, pronounced diurnal changes in malate developed. This was correlated with a progressive change in δ 13C value from about -26‰ to about -16‰ which is consistent with a change from normal C3 photosynthetic CO2 fixation to a predominantly nocturnal CO2 assimilation pattern involving Crassulacean Acid Metabolism.