Individual non-marine ostracod valves contain Sr in proportion to the Sr content of their host lake water. Two commonly coexisting, large planktonic species, Mytilocypris praenuncia and Australocypris robusta, have a Sr distribution coefficient KD (Sr/Ca) of 0.082 which is independent of temperature in the range 10-18��C. If the relationship between salinity and the Sr content of a hydrologically simple lake is known, palaeosalinities can be calculated from fossil ostracods. We present here palaeosalinities for the past 10,000 yr for Lake Keilambete, Victoria, calculated from ostracod Sr/Ca of fossil material which are in striking agreement with independent interpretations of palaeosalinity. The range of salinity spans 14-95���. We also show that a reliable quantitative continuous indicator of lacustrine palaeosalinity is important for the elucidation of continental palaeoenvironments for palaeoclimatology and archaeology and ocean-continent climatic correlations and interactions. �� 1985 Nature Publishing Group.