Several hundred specimens of carbonate rocks and shells have been analyzed for strontium by a spectrographic method which gives a precision of 5 per cent or better. The average value of Sr/1000 Ca for all limestones was 0.71. Fossils are consistently higher but vary greatly, depending on the salinity and composition of the original ocean environment, the ratio of aragonite to calcite in the original shell, and the subsequent recrystallization history. On the average, fossils contain twice as much strontium as the surrounding carbonate matrix. Constancy of strontium content of a long range genus is evidence for similar marine environment at least since the Cretaceous. Temperature of the ocean water is a relatively insignifiant factor in determining the strontium content of fossils. No evidence could be detected for seasonal fluctuations in Sr/Ca ratio.