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Thirty molluscan shells from twenty levels from the first 4.5 m of a long core drilled in the crater lake of Valle di Castiglione, near Rome, have been analysed for their oxygen and carbon stable isotope compositions. The investigated portion of the core covers the last 14.2 ka BP. The large range in delta(18)O (11 parts per thousand) suggests that hydrological conditions exercise the main control on the isotopic composition of the biogenic carbonate in the Valle di Castiglione palaeolake. Samples younger than about 5 ka show a marked O-18 depletion, which we attribute to the onset of a temperate wet climate, corresponding to the maximum of humidity. Isotopic results are in good agreement with pollen analyses and prove to be a sensitive tool for reconstructing Quaternary climatic evolution from the continental lacustrine sediments of Central Italy.