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New stable isotope analyses on molluscan shells from a long core drilled in the crater lake of Valle di Castiglione, near Rome, extended the investigated portion of the core to 37 m. The succession of δ18O‰ values in the core interval 37–2.3 m ranges from −2.8 to +6.9‰ with only six samples below 0‰ (PDB). These results point to arid climatic phases coupled with the high measured δ18O values of the biogenic carbonate. In contrast, depleted 18O samples correspond to wet climatic periods, in agreement with a strong evaporative control on the lake water isotopic composition. The 13C content of the shells shows sharp changes controlled by the dissolved inorganic carbon isotope budget. Isotopic data suggest that the whole body of water behaved as a closed system, thus resembling lacustrine systems located in arid and semiarid regions where hydrological control dominates the geochemical parameters.