Recent work has provided useful Mg/Ca to water temperature calibrations for shallow-dwelling planktonic foraminifer species. Globorotalia truncatulinoides (right coiling (R)) is a deep-dwelling species that can serve as a source of information about the temporal variability in the water characteristics of the thermocline. We present a temperature calibration for the Mg/Ca in the shell of G. truncatulinoides (R) and examine some of the practical issues associated with evaluating the usefulness of the technique. The Mg/Ca in the primary and the secondary calcite of individual G. truncatulinoides (R) correlates exponentially with water column temperatures, showing a change of ∼10% in the Mg/Ca per 1°C (R2 = 0.92). A limited comparison with plankton tow samples demonstrates that the average Mg/Ca temperature was offset +1°C from the average temperature calculated using the δ18O calibration of O'Neil et al. , and the Mg/Ca temperatures have a range similar to the δ18O temperatures. Comparisons of the [Mg] in the core top samples to water depth of deposition indicates that dissolution does not alter the measured value of Mg in the primary calcite.