The oxygen isotopic compositions of land snail shell carbonate may be used as environmental indicators. A study of a single species, Levantina caesareana, shows that modern snails from an arid zone (less than 300 mm rain) are enriched in 18O by 2‰ compared to the snail carbonate from a moderate climate zone. These measurements combined with those of the mean diameter of the shell are indicative of the climatic zone in which the snail lives. A comparison of the data on ancient populations with those on the modern one shows that the desert migrated northward in the early Holocene in Israel.