The Pleistocene coral reefs which form the upper Florida Keys are in a state of alteration from aragonite to calcite. A quantitative relationship between the Sr content of the corals and their aragonite-calcite ratios was found. Sr and Ca determination were made with a Zeiss flame photometer. Aragonite-calcite ratios were determined qualitatively with Meigen stains and quantitatively using X-ray techniques. Where the stains showed that aragonite was high relative to calcite, the Sr content was correspondingly high. Where the stains indicated that calcite was dominant, the Sr content was low. X-ray analyses confirmed these results. In several cases it was noted that within a single sample there were distinct areas which were aragonite and others which were calcite. These sections were separated and analyzed. In all such cases the aragonite portion of a sample contained about twice as much Sr as did the calcite portion of the same sample. It is suggested that Sr content is a factor which inhibits the alteration of aragonite to calcite under natural geologic conditions, and only when much of the Sr is removed may the alteration take place.