The strontium, magnesium, and calcium contents of the shells of the planktonic foraminifer Globorotalia truncatulinoides from continental rise sediments were determined by electron microprobe. This species spends the early part of its life cycle in the upper water column where it precipitates the lamellar portion of its shell. It then sinks below the permanent thermocline where it is secondarily calcified with a blocky calcite coating. This change of environments through the organism’s life allows a test of the hypothesis that an individual can change the composition of the carbonate material secreted in response to changes in the temperature of of the water mass in which it lives. All individuals except a juvenile showed decreasing magnesium contents in the outermost portion of their shells, corresponding to decreasing temperature in successively deeper water. The juvenile form showing no secondary calcification was uniform in composition. Because the Mg/Ca ratio in seawater is constant, this differentiation strongly supports the contention that an individual organism, as it precipitates shell material, can regulate its composition in response to environmental changes in temperature. No significant differences in composition were found between left- and right-coiling specimens. The difference between lamellar and blocky calcite was found to be significantly less in a sample from 10 cm below the top of the piston core than those from above or below this level. This difference is produced by a decrease in the magnesium content of the lamellar calcite and may correspond to a surface water temperature 2.5°C lower approximately 1,100 years ago.